Go to Postal2Complete\System

Open NicksCoopSave.ini and CTRL+F and search "bNightMode"

Change it from "False" to "True"

Right below bNightMode is "bNoHolidays", make sure that is set to "False"
(if you don't have the Holidays settings unlocked yet.)

Cant seem to get Night Mode to actually work even if its enabled.

Not sure if this is the same process for the Steam build.