I fairly recently got the season switcher and set out to get all of the holiday achievements. I managed to get all in one run except Spooky Cookies and Eldeer (didn't feel like doing the Grandmapocalypse). After ascending and getting Spooky Cookies, I turned on Christmas and prepared for Eldeer. Thinking it would take me centuries, I was astonished to get an Elder Frenzy/Reindeer within the first 10 minutes of Christmas.
I just wanted to share this because I would assume that the conditions for Eldeer are considerably rare and I seem to have lucked out. Pretty cool!