How can you figure out the date of Thanksgiving for any given year?

arithmetic/clock/thanksgiving New

New entry, edited from material collected for the archive and not yet in a released version.

Problem

How can you figure out the date of Thanksgiving for any given year?

Can any of you puzzle experts think of any way to figure out what day
that Thanksgiving comes on for any specified year?

-- Jeff Moser <jeff.moser@multisystem.com>

Show solution

Take the year number y.  Divide y-1900 by 4 to get q remainder r.
Divide y+q+4 by 7 to get f remainder g.  If g is less than 5, subtract
g from 5 to get d.  Otherwise, subtract g from 12 to get d.
Thanksgiving comes on d+21.

For example, the current year is 1995.  Divide 95 by 4 to get 23
remainder 3.  Divide 122 by 7 to get 17 remainder 3.  Subtract 3 from 5
to get 2.  Thanksgiving comes on 23 November 1995.

[ Editor: the example divides (y-1900)+q+4 = 95+23+4 = 122, not
y+q+4; with y = 1995, 1995+23+4 = 2022 leaves remainder 6 and gives
d+21 = 27. ]

I forgot to mention that this only works until the year 2100.

-- Brian Tung <brian@isi.edu>