Holiday Calculations?

Folks,

I'm spec'ing a calendar app for PostgreSQL, and was wondering if anyone
had already solved the following problem:

How can I calculate the dates of American holidays?

Obviously, Christmas & New Year's are easy. As is July 4.

However, Thanksgiving is the last Thursday in November, unless the month
ends on a Thursday or Friday, in which case it is the next-to-last.
Memorial Day and Labor Day are simpler, but also use the "First or Last
Monday in x month" idea.

I was wondering if anyone had already figured out these calculations, in
any language (SQL would be terrific).

Thanks!

-Josh

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