Doesn't seem to be tied to holidays, the event occurs every time for me when moving back to Kiyoko's square immediately after the housewarming scene. Tried it on a couple of new save files and its happened on both.
Huh, didn't proc for me. For me order of events was: Kiyoko is full 9 tail kitsune - Freed Kiyoko - waited for house warming scene - house warming scene - Became Kitsune myself - proposal for marriage. Never procced for me. Holiday events is off, silly mode is on, Dark difficulty.
Though it did proc immediately after going back to fox home after the wedding.
Edit: on another note this is the earliest I've ever married Kiyoko. I never free her until she's full 9 tail kitsune. Day 3 is the earliest you can start kit making and it takes minimum of 3 days per kit. So then Day 99 is the earliest in theory she could be released with all 9 of her tails. I normally average 155 but this time I did it in 133 days.