How do I make Christmas last all year?

Solution 1:

Unless someone makes a mod or you continually reset your system clock, it's not possible. As soon as you enter a world outside of the designated holiday time frame (Dec. 15th-31st), Santa will die instantly.

You can keep your access to Santa Claus by making a dedicated "Santa world" with him in it, and only entering it on special occasions when you need him (of course tampering with your system clock beforehand).

You can start the Frost Legion event outside of the holidays, but they will not drop presents, so your supply of Snow Globes will eventually run out. If your world is not in hardmode yet, keep your presents and unpack them when it is; else you will get no Snow Globes at all and the event will be unavailable.