The first anniversary event takes place long after Mayuri, one or two chapters later; you'll know because it starts with a fight between the girls. The second anniversary event, on the other hand, begins after the Mother's Day special.
I've found all the events. Thanks for your hard work, shousaka. It reminded me of the anime. Very Nostalgic. I think I've found some easter egg because I named all the girls and their codenames just like in anime. XD
oh, I hope the yellow-haired girl that I talked with her at the park bench could join the harem, too. XD