This is a LaTeX package that can calculate the day of the week given any date. The algorithm we use is the Doomsday algorithm devised by John Conway.
I have always wanted to improve my diary files. This package is one of the easier components of the big plan. After I wrote commalist, I became interested in making my commands fully expandable. The commands in doomsday are the outcome of my first exercises. Therefore, they are necessarily a bit rough, but they should work. In case you care: all but one commands in doomsday can be completely expanded to the correct answer.
The current version of doomsday is 1.0, which is dated 7 April, 2012.