For the method/assembly to figure out US Holidays, basically just figure out all the major holidays and the "formula" that they use.
For the ones that never change, like Christmas, it's easy - December 25th.
For the ones that do change somewhat, there's usually a formula - like the third Monday in February being Presidents Day. You can just have the method figure this out for a given year.
This won't work for holidays without any particular pattern (i.e., some committee decides what the date is every year) but for all the major ones there's easily discernible formulas.
This would actually be a great candidate for Test Driven Design. You will know all of the major holiday dates for a particular year, so you should be able to feed that year into the method and get the right answers.
