You could just as well say "Have a great time!"
It is usually said about parties or activities where you would expect to have a very good, noisy time or a very rewarding, memorable one (or both).
It does not necessarily imply drinking, but it would definitely be used at parties where a lot of noise and uninhibitied acitivies were going to take place.
However, it can just as well be used at an activity that you would have some excitement and fun - it doesn't have to mean drinking are acting foolishly.