For those hosting their own servers using ASM, here's what you'll want to drop into your ADMINISTRATION tab:
The first epoch time is set to 00:01 CST Jan 1, 2021, and the second is 12:00 CST Jan 1, 2021. You can change these using the epoch converter provided in WC's post: https://www.epochconverter.com/
?NewYear1UTC=1609480860?NewYear2UTC=1609524000
If you're bare-hosting, drop this into your game.ini:
-NewYearEvent?NewYear1UTC=1609480860?NewYear2UTC=1609524000
I believe this should work the same for Singleplayer, but don't quote me on it. Most people I know will use ASM to host their SP experience, since it gives you far greater control over the game versus the built-in tools 👍