The end dimension gets reset every server reboot. The resets on the other extra dimensions are performed automatically with the first restart on the first day of the month.
The First Day of the month being Midnight Central European Time for the EU servers and Midnight Eastern Standard Time for the NA servers. To be safe, REMOVE your items a day before and replace a day after as there will be NO REFUNDS or discussion due to items lost during resets.
Here is a list of worlds NOT affected by these resets: