Personally, I would schedule my session for as soon as the event was over. I would want to be able to decompress with my T right away. I think it would be useful for T to hear about the situation (and my response to the situation) while I was still in the reaction-- before I calmed down. I would not choose to have my session right before the event, because I would not want to upset myself beforehand-- I would want to just "get through it" and then deal with it, with T, as soon as it was over. But that's how I work; others may work differently.