On the 1st day only and using pure guesswork based on my experiences of previous LS first showings, the audience will be made up of 40% hardcore fans, 40% general X-Men film lovers and 20% general film fans (who aren't fans outside of the X-films and are just going for a good day out or with someone else who loves the films). I would expect 99% of the audience to have seen and really liked the 1st 2 films.
After Day 1 things will change totally and the atmosphere won't be comparable. Also, I wouldn't attend an afternoon showing if there is one.