What a fun question :)
-President's Day - serve foods associated with particular presidents, ask guests to come dressed as a president
-St. Patrick's Day - prepare traditional foods, watch Irish movies - The Quiet Man
Also, just about every day of the year has some sort of celebration attached to it. For example, National Doughnut Day is in June is historically associated with the Salvation Army. Purchase plain doughnuts and lots of goodies such as frosting, sprinkles, etc. and let guests decorate them. Hold contests and have guests vote on the most creative, most over- the-top, most beautiful, etc. dougnuts and give little prizes. Ask guests to contribute a small cash donation for the Salvation Army in honor of your party.