Before a woman gets married she goes out with all her female friends on a HEN DO (she being the hen). The man has a STAG do (him being the stag).
It's supposed to be their last night of freedom before their wedding.
A lot of people now go abroad for theirs, so for a stag or hen do, you'd expect lots of (stereotypically) rowdy men/women, good fun as long as you know how to handle them.
Hope that helps
Ezy