If 'pre' is previous, 'post' is after, and 'peri' is current, what is "initiation"?
Solution 1:
I don't think there's a simple affix that has this meaning. When one wants to form a word for this, it's common to use the compound prefix "start-of-". So one might say
The start-of-game activities in a foodball game are the coin toss and kickoff.