A sexual encounter that takes place during a night of carousing, or otherwise drunken revelry.
Orgies not mutually exclusive.
Marmaduke: 'You are cordially invited to attend an evening of Bach, cribbage, and orgiastic festivity, on the 14th day of May. Arrive at 4:45 in the post meridian, and carriages for no later than dawn.'
Fitzwilliam: 'No doubt another one of your choir, cards and chains evenings?'
Marmaduke: 'Yes, the usual merrymating.'