						M O N T H
						Jan	Apr	Sep	Jun	Feb	Aug	May
						Oct	Jul	Dec		Mar	Feb
							Jan			Nov
DAY OF THE MONTH	1	8	15	22	29	A	B	C	D	E	F	G	Monday	DAY OF THE WEEK
	2	9	16	23	30	G	A	B	C	D	E	F	Tuesday
	3	10	17	24	31	F	G	A	B	C	D	E	Wednesday
	4	11	18	25		E	F	G	A	B	C	D	Thursday
	5	12	19	26		D	E	F	G	A	B	C	Friday
	6	13	20	27		C	D	E	F	G	A	B	Saturday
	7	14	21	28		B	C	D	E	F	G	A	Sunday
							1968	1969	1970	1971		1972
						1973	1974	1975		1976	1977	1978
						1979		1980	1981	1982	1983
						1984	1985	1986	1987		1988	1989
						1990	1991		1992	1993	1994	1995
							1996	1997	1998	1999		2000
						2001	2002	2003		2004	2005	2006
						2007		2008	2009	2010	2011
						2012	2013	2014	2015		2016	2017
						2018	2019		2020	2021	2022	2023
							2024	2025	2026	2027		2028
						2029	2030	2031		2032	2033	2034
						2035		2036	2037	2038	2039
						2040	2041	2042	2043		2044	2045
						2046	2047		2048	2049	2050	2051
							2052	2053	2054	2055		2056
						2057	2058	2059		2060	2061	2062
						2063		2064	2065	2066	2067
						2068	2069	2070	2071		2072	2073
						Y E A R

How do I use Herschel's Perpetual Calendar?

Use the Day Number and Month together and also the Day of Week and Year of Century together. The two are linked by the same letter in the body of the table.
Leap years are shown as 2012 and you must use the columns with Jan and Feb for such years.

Here are some examples:

On what day of the week was July 4 1998?

Use the Day Number 4 and Month July to find a letter from the table: F).
Use the Year 1998 and the letter F to find a row for the Day of Week. The row for F in the column above 1998 is labelled Saturday.
So July 4 1998 is a Saturday.

Which months in 2004 have a Friday 13th?

Use the Day of Week Friday and Year 2004 to find a letter from the table: A.
Use the Day Number 13 and find the letter A in its row. The appropriate column is headed Aug, Feb.
Since 2004 is a leap year (it is shown as 2004 and not as 2004) then Feb is valid as February of a leap year.

The 13th is a Friday only in February and August in 2004.

On which years will my birthday, 4 May, fall on a Saturday?

We know the Day Number 4 and the Month May so use these to look up a letter in the table: D.
Use the row labelled Saturday and locate the letter D in it.
The years under the appropriate column are: ..., 1968, 1974, 1985, 1991, 1996, 2002, 2013, 2019, ...
My birthday (4 May) will fall on a Saturday in 2013, 2019 ...

But I thought 2000 was not a leapyear?

There seems to be an erroneous rule about leapyears that is found on some pages on the Web and in some software. The correct rule is

that a leapyear is a year which is an exact multiple of 4
but a new century year (..., 1900, 2000, 2100, ...) must also be a multiple of 400 too.

Eric's Treasure Trove of Science has an entry under The Gregorian Calendar which has some more historical information on calendars.

The Julian Calendar

The old Julian Calendar had been commissioned by Julius Caesar and dated from BC446 based on research undertaken by the Egyptian Sosigenes. The calendar until then had 10 months of 30 days each. We still see the remnants of this in the names of the months:

September (7), October (8), November (9) and December (10) Julius Caesar introduced a year of 365 days with two extra months named after himself (July) and the first Roman emperor Augustus Caesar (August). Augustus insisted that his month had the same number of days as Julius's (31 days each) and so February was shorted to accommodate this to make 365 days in total.
It also introduced the rule that every fourth year was to be a leapyear with the simple rule that every year which is a multiple of 4 has an extra day (a leapyear). The reason for this is that there are not exactly 365 days in a year - the time it takes the earth to revolve around the sun in its orbit - but there are roughly 365.25 days so we need to catch up on the extra 0.25 (=1/4) of a day by adding in a whole day every 4 years.

The Gregorian Calendar

In fact, even this figure of 365.25 is not accurate enough - it is too big by about 0.0078 days (that's about 5 mins 37 secs).
This difference was noticed by astronomers and by 1582 when the date on the calendar was out of line with the date "in the stars" by about 10 days, Pope Gregory decided to correct it. He ordained that October 5th 1582 should be called October 15th to remove the extra 10 days. Also, since 0.0078 is quite close 0.0075 which is is equal to the handy fraction 3/400, then 3 days in every 400 should not be leapyears. Gregory's system incorporated this in the simple rule that loses 3 years in every 400: the new century years must be multiples of 400 to be leapyears.
Thus 1800 and 1900, although divisible by 4 are not leapyears, and neither are 2100, 2200 and 2300. On the other hand, the year 2000, being a multiple of 400 is a leapyear.

This system is called the Gregorian Calendar in honour of Pope Gregory and is now used worldwide.
Italy, France, Spain and Portugal adopted Gregory's new calendar in 1582 and other Catholic countries followed later. In Great Britain, it was adopted in 1752 and by then 12 days had to be removed from the calendar, not 10, in September, causing riots in the streets of London because people thought they had lost 12 days of their lives!
Mark Bader has the details from the original 1751 Act of Parliament under King George II bringing in the Gregorian calendar from September 1752 in the UK and "his Majesty's Dominions...".
This century has seen its adoption in China, Bulgaria, Turkey, Romania and the former USSR and Yugoslavia. The latest country to adopt it was Greece in 1923.

We still see remnants of the old calendar in that Christmas Day has retained its former "orthodox" date and is 12 days out - being celebrated on January 6th instead of December 25th in some parts of Europe (the twelfth day of Christmas).