Got a contract?
Have a decent p.a.?
How many songs do you know now?
What type of music are they expecting?
Venue indoors/outdoors?
Will you have to keep the volume down because it's a social?
Take a look at:
If only I knew then what I know now... 25 Golden Rules for Bands
"since most average songs are roughly 3 to 6 minutes in length your looking at 10 to 20 songs per set...."
An easy way around having to learn so many songs and to satisfy the crowd that likes to dance it to extend each song by playing it all the way through twice(or some such variation therein).