This is a neat little game in which you homebrew a winter solstice holiday. It's got a bit of a ritual feel to it, like something out of Playing Nature's Year, but it's definitely not crunchy in any way.
I think I would either use this to to create a holiday with a close-knit friend group, one that we're at least sort of planning to observe, or as sort of a session zero for a fantasy campaign that starts during the winter. Honestly, I think a dnd game would get some significant mileage out of inventing a small town's festivities (complete with food and drinks,) and then the next session the place gets attacked by midwinter leprechauns or whatever, and now there's suddenly a kind of automatic emotional attachment to the campaign hook.
To be clear, I don't think that's what Festival Of Deep Winter was written for, but it's also not something I think I'd play just to play. It feels a bit more like it's a toolkit, and like it should be used for a purpose.
Overall, I'd recommend this to anyone who wants a calm, extremely seasonal game.
With a few minor modifications, I suspect it'd work for fall, winter, or spring too.