The case could probably be made that only in America do we concern ourselves with celebrating this kind of stuff (and then we can't decide whether to start with year zero or year one!) -- anyway, since we don't have anything that's 250 years old yet, let me be the first to coin the word semiquincentennial (I like that a bit better than quinsemicentennial.)


EDITED NOTE: I, of all people, should know better than to assume that any idea is new:
http://www.sls.lib.il.us/reference/por/features/97/anniverl.html