Every Wednesday at Six
Poker nights were Wednesdays, or else the times that the ship's clock stated were Wednesdays at 6:00 PM, and they were incredibly secret. So much so that the only four people who knew about them were Sulu, Chekov, Scotty and the doctor. They made a very strong point of not inviting Jim and it never even crossed their minds to invite Commander Spock. This was a time reserved for the sane crewmembers aboard the ship. As such women were not invited, and no one who wasn't single was invited. And Jim didn't qualify for sanity because he was just Jim.
In general they each had their own thing to bring to the table. Chekov brought the cards and chips, Scotty brought the interesting stories, Sulu brought his different variants of the game that they played and McCoy brought the alcohol – which brought the others to praise him as their hero, and him to sometimes question why he still came every Wednesday.
Single sessions lasted about two hours before they devolved into drunken shouting matches between McCoy and Scotty over who won the last game or a political debate that Ensign Chekov started that really had nothing to do with poker whatsoever. There was no set end-time though, it usually just stopped when the game was less important than all the side-chatter or they were out of booze – which happened at least every other time.
They didn't allow women, but they sure talked about them, and sometimes accusations were made against other members regarding their status with a crewmember of the opposite sex and allegiances to the 'Circle of the Cardplayers of the Enterprise' (as Scotty dubbed it) were called into question. Once Sulu was shunned for three weeks straight before they discovered that Yeoman Rand was only delivering him his lunch every day at noon because she owed him money.
But no matter how different they were or how often they fought, their love of a good time kept them together.
That, and their funny stories about the Captain.