Jan = Jan in a leap year,     Feb = Feb in a leap year
Jan = Jan in a non-leap year, Feb = Feb in a non-leap year
                       M  O  N  T  H
               Jan  Apr  Sep  Jun  Feb  Aug  May   /
               Oct  July Dec       Mar  Feb      /
DAY OF MONTH        Jan            Nov          /     DAY OF WEEK
1  8 15 22 29   A    B    C    D    E    F    G      Monday
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
              /      68   69   70   71        72  
             /  73   74   75        76   77   78 
            /   79        80   81   82   83           1900s
                84   85   86   87        88   89
                90   91        92   93   94   95
                     96   97   98   99        00      ......
                01   02   03        04   05   06
                07        08   09   10   11           2000s
                12   13   14   15        16   17 
                18   19        20   21   22   23
                            Y E A R        o f        C E N T U R Y

Summary of how to 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 indicated in bold eg 92, 96, 00, 04 etc meaning 1992, 1996, 2000 and 2004.

Here are some examples:

On what day of the week is July 4 1998?

Use the Day Number (4) and Month (July) to find a letter from the table (it is F).
Use the Year of the Century (98) and the letter F to find a row for the Day of Week. The row for F in the column above 98 is labelled Saturday.
So July 4 1998 is a Saturday.

Which months this year (1998) have Friday 13th?

Use the Day of Week (Friday) and Year of Century (98) to find a letter from the table (G).
Use the Day Number (13) to find the letter ( G) in its row. The appropriate column is headed feb, Mar, Nov.
Since 1998 is not a leap year (it does NOT follow a space in the list of years below the table) then feb is valid as Februrary of a non-leap year.

The 13th will fall on a Friday in February, March and November in 1998

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:
..., 68, 74, 85, 91, 96, 02, 13, 19, ...

My birthday (4 May) will fall on a Saturday in 2002, 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 commisioned 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)
Julias Caesar introduced a year of 365 days with two extra months named after himself (July) and the first Roman emporer 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 divisble 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 Pop eGregory and is now used worldwide.
Italy, France, Spain and Portugal adopted Gregory's new calendar in 1582 and other Catholic countires followed later. In Great Britain, it was 1752 when it was adopted and by then 12 days had to be removed from the calendar, 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 bringing in the Gregorian calendar from September 1752 in the UK under King George II 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 (St Nicholas 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).

More Links

There are more details and many more links on The Home Page for Calendar Reform.

Dr Ron Knott (email: R.Knott@surrey.ac.uk)
May 1998 (updated August 2000)