You can override the active holiday in the launch options as if it were a different time of year:
- Right-click Unturned in your Steam Library
- Select Properties... to open the General tab
- Find the Launch Options field
- Type "-Holiday=XMAS" without quotes
Valid options other than XMAS are Halloween, HW, Christmas, PrideMonth, Valentines, and AprilFools.
Can I play with a different holiday in multiplayer?
In multiplayer the active holiday is controlled by the server. If you are hosting the server you can add the "-Holiday=XYZ" option to the server's command-line arguments to override the default.