I recall someone with official status saying that the intellectual property rights for Turkor and Lepus, et cetera, belong to the company that developed the console/mobile versions from that era. If someone were to mod Terraria with their own holiday bosses and items that didn't violate IP law, of course that would be legal.
Thanksgiving in the United States takes place on the fourth Thursday in November. Sometimes that's the last Thursday, but in some years November has five Thursdays.
Easter's date can be calculated without lookup tables by applying any of several algorithms, using only basic arithmetic operations. I use the method described in Samuel Butcher's "Ecclesiastic Calendar: Its Theory And Construction", published in 1877.
If anyone is interested, I have a version adapted for PHP, which should be easy to translate into C#. Might have a C# version already written; will leave grep searching for it.
Hope this helps.