questions

Are the tournaments every month on a set day or does it change from one month to the next?

The last one was on the 25th of May. So it's the same day so far. It's hard to tell without documentation of who won the last tourneys. I had to find a bot that I knew won a category :/

Tournaments are always on the 25th of the month.

As for viewing results for older tournaments, you can click the [view results] link on the homepage and then manipulate the URL, e.g. change the 5 to 4 on this link:

http://bots4.net/tournament/edition-5

That's far from ideal of course and I intend on giving tournaments a more easily-navigable interface for historical results.