Determination of the day of the week

Corresponding days

Every seventh day in a month has the same name as the previous:

Corresponding months

"Corresponding months" are those months within the calendar year that start on the same day of the week. For example, September and December correspond, because September 1 falls on the same day as December 1. Months can only correspond if the number of days between their first days is divisible by 7, or in other words, if their first days are a whole number of weeks apart. For example, February of a common year corresponds to March because February has 28 days, a number divisible by 7, 28 days being exactly four weeks. In a leap year, January and February correspond to different months than in a common year, since adding February 29 means each subsequent month starts a day later.

The months correspond thus:
For common years:

• January and October.
• February, March and November.
• April and July.
• No month corresponds to August.

For leap years:

• January, April and July.
• February and August.
• March and November.
• No month corresponds to October.

For all years:

• September and December.
• No month corresponds to May or June.

In the months table below, corresponding months have the same number, a fact which follows directly from the definition.

Corresponding years

There are seven possible days that a year can start on, and leap years will alter the day of the week after February 29. This means that there are 14 configurations that a year can have. All the configurations can be referenced by a dominical letter, but as February 29 has no letter allocated to it a leap year has two dominical letters, one for January and February and the other (one step back in the alphabetical sequence) for March to December. For example, 2018 is a common year starting on Monday, meaning that 2018 corresponds to the 2007 calendar year and with the last 10 months corresponds to the 2012 calendar year. On the other hand, 2019 is a common year starting on Tuesday, meaning that 2019 corresponds to the 2013 calendar year. 2020 is a leap year starting on Wednesday, meaning that 2020 corresponds to the 1992 calendar year, meaning that the first two months of the year begin on the same day as they do in 2014 (i.e. January 1 is a Wednesday and February 1 is a Saturday) but because of a leap day the last ten months correspond to the last ten months in 2015 (i.e. March 1 is a Sunday to December 31 is a Thursday.). 2021 is a common year starting on Friday, meaning that 2021 corresponds to the 2010 calendar year and with the first 2 months corresponds to the 2016 calendar year. 2022 is a common year starting on Saturday, meaning that 2022 corresponds to the 2011 calendar year and with the last 10 months corresponds to the 2016 calendar year. For details see the table below.

Notes:

• Black means the all months of Common Year
• Red means the first 2 months of Leap Year
• Blue means the last 10 months of Leap Year

Corresponding centuries

See the table below.

The Julian starts on Thursday and the Gregorian on Saturday.

Complete table: Julian and Gregorian calendars

For Julian dates before 1300 and after 1999 the year in the table which differs by an exact multiple of 700 years should be used. For Gregorian dates after 2299, the year in the table which differs by an exact multiple of 400 years should be used. The values "r0" through "r6" indicate the remainder when the Hundreds value is divided by 7 and 4 respectively, indicating how the series extend in either direction. Both Julian and Gregorian values are shown 1500–1999 for convenience. Bold figures (e.g., 04) denote leap year. If a year ends in 00 and its hundreds are in bold it is a leap year. Thus 19 indicates that 1900 is not a Gregorian leap year, (but 19 in the Julian column indicates that it is a Julian leap year, as are all Julian x00 years). 20 indicates that 2000 is a leap year. Use Jan and Feb only in leap years.

For determination of the day of the week (1 January 2000, Saturday)

• the day of the month: 1 ~ 31 (1)
• the month: (6)
• the year: (0)
• the century mod 4 for the Gregorian calendar and mod 7 for the Julian calendar DW: (Sa)
• adding Sa + 1 + 6 + 0 = Sa + 7. Dividing by 7 leaves a remainder of 0, so the day of the week is Saturday.

For determination of the dominical letter (2000, BA)

• the century DL: (A)
• the year: (0)
• subtracting A - 0 = A.

For determination of the doomsday (2000, Tuesday)

• the century DD: (Tu)
• the year: (0)
• adding Tu + 0 = Tuesday.

Revised Julian calendar

Note that the date (and hence the day of the week) in the Revised Julian and Gregorian calendars is the same from 14 October 1923 to 28 February AD 2800 inclusive and that for large years it may be possible to subtract 6300 or a multiple thereof before starting so as to reach a year which is within or closer to the table.

To look up the weekday of any date for any year using the table, subtract 100 from the year, divide the difference by 100, multiply the resulting quotient (omitting fractions) by seven and divide the product by nine. Note the quotient (omitting fractions). Enter the table with the Julian year, and just before the final division add 50 and subtract the quotient noted above.

Example: What is the day of the week of 27 January 8315?

8315-6300=2015, 2015-100=1915, 1915/100=19 remainder 15, 19x7=133, 133/9=14 remainder 7. 2015 is 700 years ahead of 1315, so 1315 is used. From table: for hundreds (13): 6. For remaining digits (15): 4. For month (January): 0. For date (27): 27. 6+4+0+27+50-14=73. 73/7=10 remainder 3. Day of week = Tuesday.

Dominical Letter

To find the Dominical Letter, calculate the day of the week for either 1 January or 1 October. If it is Sunday, the Sunday Letter is A, if Saturday B, and similarly backwards through the week and forwards through the alphabet to Monday, which is G.

Leap years have two Sunday Letters, so for January and February calculate the day of the week for 1 January and for March to December calculate the day of the week for 1 October.

Leap years are all years which divide exactly by four with the following exceptions:

In the Gregorian calendar – all years which divide exactly by 100 (other than those which divide exactly by 400).

In the Revised Julian calendar – all years which divide exactly by 100 (other than those which give remainder 200 or 600 when divided by 900).

The following table gives the letter for every day of the year:

Note that in the calendar of the Church of England 29 February of a leap year does not have a letter.

Gauss's algorithm

In a handwritten note in a collection of astronomical tables, Carl Friedrich Gauss described a method for calculating the day of the week for 1 January in any given year.^[5] He never published it. It was finally included in his collected works in 1927.^[6]

Gauss' method was applicable to the Gregorian calendar. He numbered the weekdays from 0 to 6 starting with Sunday. He defined the following operation: The weekday of 1 January in year number A is^[5]

where is the remainder after division of y by m,^[6] or y modulo m.

This formula was also converted into graphical and tabular methods for calculating any day of the week by Kraitchik and Schwerdtfeger.^[6][7]

Zeller’s algorithm

In Zeller’s algorithm, the months are numbered from 3 for March to 14 for February. The year is assumed to begin in March; this means, for example, that January 1995 is to be treated as month 13 of 1994.^[8] The formula for the Gregorian calendar is

where

• Y is the year minus 1 for January or February, and the year for any other month
• y is the last 2 digits of Y
• c is the first 2 digits of Y
• d is the day of the month (1 to 31)
• m is the shifted month (March=3,...January = 13, February=14)
• w is the day of week (1=Sunday,..0=Saturday)

The only difference is one between Zeller’s algorithm (Z) and the Gaussian algorithm (G), that is Z − G = 1 = Sunday.

(March = 3 in Z but March = 1 in G)

So we can get the values of months from those for the Gaussian algorithm by adding one:

Schwerdtfeger's method

In a partly tabular method by Schwerdtfeger, the year is split into the century and the two digit year within the century. The approach depends on the month. For m ≥ 3,

so g is between 0 and 99. For m = 1,2,

The formula for the day of the week is^[6]

where the positive modulus is chosen.^[6]

The value of e is obtained from the following table:

The value of f is obtained from the following table, which depends on the calendar. For the Gregorian calendar,^[6]

For the Julian calendar,^[6]

Lewis Carroll's method

Charles Lutwidge Dodgson (Lewis Carroll) devised a method resembling a puzzle, yet partly tabular in using the same index numbers for the months as in the "Complete table: Julian and Gregorian calendars" above. He lists the same three adjustments for the first three months of non-leap years, one 7 higher for the last, and gives cryptic instructions for finding the rest; his adjustments for centuries are to be determined using formulas similar to those for the centuries table. Although explicit in asserting that his method also works for Old Style dates, his example reproduced below to determine that "1676, February 23" is a Wednesday only works on a Julian calendar which starts the year on January 1, instead of March 25 as on the "Old Style" Julian calendar.

Algorithm:^[9]

Take the given date in 4 portions, viz. the number of centuries, the number of years over, the month, the day of the month. Compute the following 4 items, adding each, when found, to the total of the previous items. When an item or total exceeds 7, divide by 7, and keep the remainder only.

Century-item: For `Old Style' (which ended 2 September 1752) subtract from 18. For `New Style' (which began 14 September 1752) divide by 4, take overplus from 3, multiply remainder by 2.

Year-item: Add together the number of dozens, the overplus, and the number of 4s in the overplus.

Month-item: If it begins or ends with a vowel, subtract the number, denoting its place in the year, from 10. This, plus its number of days, gives the item for the following month. The item for January is "0"; for February or March, "3"; for December, "12".

Day-item: The total, thus reached, must be corrected, by deducting "1" (first adding 7, if the total be "0"), if the date be January or February in a leap year, remembering that every year, divisible by 4, is a Leap Year, excepting only the century-years, in `New Style', when the number of centuries is not so divisible (e.g. 1800).

The final result gives the day of the week, "0" meaning Sunday, "1" Monday, and so on.

Examples:^[9]

1783, September 18

17, divided by 4, leaves "1" over; 1 from 3 gives "2"; twice 2 is "4". 83 is 6 dozen and 11, giving 17; plus 2 gives 19, i.e. (dividing by 7) "5". Total 9, i.e. "2" The item for August is "8 from 10", i.e. "2"; so, for September, it is "2 plus 31", i.e. "5" Total 7, i.e. "0", which goes out. 18 gives "4". Answer, "Thursday".

1676, February 23

16 from 18 gives "2" 76 is 6 dozen and 4, giving 10; plus 1 gives 11, i.e. "4". Total "6" The item for February is "3". Total 9, i.e. "2" 23 gives "2". Total "4" Correction for Leap Year gives "3". Answer, "Wednesday".

Since 23 February 1676 (counting February as the second month) is, for Carroll, the same day as Gregorian 4 March 1676, he fails to arrive at the correct answer, namely "Friday," for an Old Style date that on the Gregorian calendar is the same day as 5 March 1677. Had he correctly assumed the year to begin on the 25th of March, his method would have accounted for differing year numbers - just like George Washington's birthday differs - between the two calendars.

It is noteworthy that those who have republished Carroll's method have failed to point out his error, most notably Martin Gardner.^[10]

In 1752, the British Empire abandoned its use of the Old Style Julian calendar upon adopting the Gregorian calendar, which has become today's standard in most countries of the world. For more background, see Old Style and New Style dates.

Implementation-dependent methods

In the C language expressions below, y, m and d are, respectively, integer variables representing the year (e.g., 1988), month (1-12) and day of the month (1-31).

        (d+=m<3?y--:y-2,23*m/9+d+4+y/4-y/100+y/400)%7  

In 1990, Michael Keith and Tom Craver published the foregoing expression that seeks to minimise the number of keystrokes needed to enter a self-contained function for converting a Gregorian date into a numerical day of the week.^[11] It preserves neither y nor d, and returns 0 = Sunday, 1 = Monday, etc.

Shortly afterwards, Hans Lachman streamlined their algorithm for ease of use on low-end devices. As designed originally for four-function calculators, his method needs fewer keypad entries by limiting its range either to A.D. 1905-2099, or to historical Julian dates. It was later modified to convert any Gregorian date, even on an abacus. On Motorola 68000-based devices, there is similarly less need of either processor registers or opcodes, depending on the intended design objective.^[12]

    dayofweek(y, m, d)	/* 1 <= m <= 12,  y > 1752 (in the U.K.) */
    {
        static int t[] = {0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4};
        y -= m < 3;
        return (y + y/4 - y/100 + y/400 + t[m-1] + d) % 7;
    }

    dow(m,d,y) { y-=m<3; return(y+y/4-y/100+y/400+[m]+d)%7; }

                      Base = 7n + k       (i)

int daystotal (int y, int m, int d)
{
	static char daytab[2][13] =
	{
		{0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31},
		{0, 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}
	};
	int daystotal = d;
	for (int year = 1 ; year <= y ; year++)
	{
		int max_month = ( year < y ? 12 : m-1 );
		int leap = (year%4 == 0);
		if (year%100 == 0 && year%400 != 0)
			leap = 0;
		for (int month = 1 ; month <= max_month ; month++)
		{
			daystotal += daytab[leap][month];
		}
	}
	return daystotal;
}