What a great time for this thread, because Alpha Phi was founded on September 30, 1872 at Syracuse University in New York.
Edited to add: Oops, I'm getting ahead of myself! We were founded on 9/30, but now celebrate Founders' Day on October 10 as we have since 1904 when the date was changed at Convention to accommodate schools with later starting dates. You can tell I'm just looking for an excuse to celebrate twice!