Tomorrow is the hen night. Why not the doe night?
A quick Google only came up with this possible explanation on The Phrase Finder's bulletin board.
'A typical flock of chickens consisted of many hens and one rooster, if the owners were interested in breeding chicks, or many hens and no rooster if the goal was egg production. So hens hung out together, pecking and gabbling and roosters were solitary.'
Any one have any better suggestions?