This app was mentioned in 6 comments, with an average of 2.00 upvotes
I usually put up this live wallpaper . It's beautiful and very customizable. You can bring your own photos as well and messages.
https://play.google.com/store/apps/details?id=com.dualboot.apps.christmas&hl=en
I always put this on my Android devices around October.
https://play.google.com/store/apps/details?id=com.dualboot.apps.christmas&hl=en
Especially the outdoor one. I fiddle with it so it's just plain winter, then I can add Christmas stuff.
Google's Santa Tracker app is good as it has little games and stuff in it.
On iOS there's an Advent Calendar from many years ago. Don't know if you can still buy it but that stays on my iPod Touch all year round.
I use this https://play.google.com/store/apps/details?id=com.dualboot.apps.christmas&hl=en for several years now. Fun little live background. Lots of customization despite a less than easy to use settings page.
I do - but generally only at this time of year - when I use this one https://play.google.com/store/apps/details?id=com.dualboot.apps.christmas&hl=en
My Christmas recommendation is https://play.google.com/store/apps/details?id=com.dualboot.apps.christmas I bought it many years ago and supports MANY customisations