They definitely make the spells drop rate at lore much higher for it during the holiday season. I have managed to get the spell on two of my characters and one I wasn't even attempting to get that particular spell for. Just keep trying and eventually you'll get lucky.
I got headless horsemen 30 minutes in but i'm 5 hours in with no luck for reindeer yet. I hope they didn't change it so the spell isn't increased chance anymore and they wan't us to collect the spellement now. I haven't gotten a single reindeer spellement yet just a bunch of krampus.
I got Reindeer Knight with my main death wiz within a few hours during the event, which is a big kick in the teeth for me cause I wanted the regular Deer Knight. On top of that, I got Lord of Night, Krampus, and Samoorai within that few hours as well. I guess the drop rate is pretty random, but as you get more spells, the harder it is to find new ones, cause less card pool drops=less chance to find any sort of spell.
The numbers exactly, however, are hard to tell. Lydia Greyrose on the message boards stated that the numbers could be anything between 10 battles and 10,000 battles, so the odds of finding Reindeer Knight before finding other spells could be 1/n where n=some massive number. Even then, you're bound to find something else before Reindeer Knight.
With that in mind, its entirely possible to find ALL spells at once, the chances are immensely low, but it has happened at least once!