First off, nothing too forte.
Then consider the sort of people likely to be attending, are they even going to be listening to the music? Dancing? Is the party itself part of a celebration of something to do with music?
For example; In an Irish bar i used to work in I made two playlists for St. Patricks day, one for the "All age" crowd that expected to see early on in the day - which included alot of Irish Trad. and 90's/early 00's Irish chart music - and one for the younger crowd I expected to see later on in the evening, this one was a blend of lively current chart music, Irish Trad. and modern interpretations of Irish Trad. (in a variety of genres ranging from dance to pop-punk).
Both went down pretty well but I certainly couldn't have mixed the timings of the playlists; Drunken Lullabies by Flogging Molly just wouldn't have worked with the older crowd.