How come there are performances before opening night? Are they like practice?
And when do tickets usually go on sale for such things?
Those are excellent questions. Yes, these are preview performances and are customary before a big opening. These previews give the actors the opportunity to perform the play in front of an audience and the production company the chance to work out any bugs before the media review the play. the media will attend on the official opening night.
Often it is easier to get tickets to preview performances, but I think in the case of War Horse, because of the advance word-of-mouth, even the previews will be sold out!