The name given to the day before a house party otherwise know as a session or a 'sesh'
"Hey guys, it's Seshmas Eve!!!"
The night before the sesh. (Generally falls on a Thursday/Friday in the UK) The pinnacle of drinking excitement. Everyone waits excitedly to see what father Seshmas will bring them the next day.
“I’ve left some jäger bombs out for father seshmas and a doner kebab for his deer, it is Seshmas eve after all!”
Normally a day within a few weeks of Christmas involving a pub crawl and the consumption of large amounts of alcohol. A festive sesh. Traditionally at least 12 pints are drunk on the day. Even by the lightweights of the group.
Are you coming to seshmas this year Chris? It would be nice to see you make an appearance.