Does Kiyoko's Mother's Day Event happen multiple times in May or only once per year?

[213Warrior]

It was nice and wholesome, but I was wondering if my computer's clock would have to be set for next year to do it again?

 

[The Observer]

Once a year per save.

 

[wery12345]

Whats required to trigger it, cause it seems to not want to for me?