Mo,
The official session venues in the programme are only half the story - there are also usually sessions in the Tap and Spile back room, the Board (new venue last year), the Elsinore, the Ship and sometimes the Little Angel. The first two tend to be mixed, the Elsinore English, the last two Irish. Sessions happen in other pubs as well, you just need a few people and an empty room! I find that afternoon sessions are more laid back than evenings - could it have something to do with the amount of beer consumed?! For the official sessions, the best clue as to repertoire is just to read the leaders' blurb in the programme.