Every Boston Marathon is special, of course, but recent years have seen
some particularly notable races: the 100th running, the first running of
the new millenium (in 2000), and, for millenium purist die-hards, the first
running of the new millenium this April.

This year's edition is special for another reason: the 2001 marathon occurs 
the day after Easter. Exactly how rare is this? When will be the next time
it occurs?

The rule for determining the date of the marathon is simple. It is always
held on Patriot's Day, a holiday in certain parts of New England (including
Massachusetts) which commemorates the Battle of Lexington and Concord. 
The battle occured on April 19, 1775 (3 days after Easter,) but the
date for Patriot's Day has been fixed as the 3rd Monday of April to avoid
the inconvenience of a moving holiday.

The rule for determining Easter is much more complex. Roughly speaking,
it is the first Sunday after the first full moon occuring on or after the
first day of Spring.  Claus Tondering gives an excellent account of the 
intracacies of calculating the date of Easter in the Calendar FAQ 
(via ftp from rtfm.mit.edu.) Also see Jean Meeus, Astronomical Algorithms, 
Willmann-Bell, Richmond, 1991. Suffice it to say that the pattern is so
complex that it only repeats after a period of 5,700,000 years!

Here, for the record, are all the years of the current century when the
Boston Marathon occurs on the day after Easter. 

Year   Patriot's Day
====   =============
2003   4/21
2006   4/17
2014   4/21
2017   4/17
2022   4/18
2025   4/21
2028   4/17
2033   4/18
2044   4/18
2047   4/15
2049   4/19
2055   4/19
2058   4/15
2060   4/19
2063   4/16
2069   4/15
2071   4/20
2074   4/16
2076   4/20
2082   4/20
2085   4/16
2087   4/21
2090   4/17
2096   4/16
2098   4/21

During the years 2001-100000, the marathon will occur on the day after
Easter 23,542 out of 98000 times, or about 24% of the time.

Here are some related questions to ponder during your next long run. The
first three are very easy; the rest, less so.

1) What is the latest possible Marathon date?
2) What is the earliest possible Marathon date?
3) What is the most common Marathon date?

4) What is the latest possible date of Easter?
5) What is the earliest possible date of Easter?
6) What is the most common date of Easter?

7) What is the most common date on which the Marathon occurs the day 
after Easter?