Holidays randomly disappear and I can't add them back to days with events

Harvestfest and Love Day should reschedule themselves the next time their seasons come around if you haven't deleted them from the save. You could try changing the season length in Settings if you want them back the same season. (I haven't tested any of this, so I hope it works.) 

It would probably help if you can identify a cause for your holidays going missing. Good luck.