Christmas

Christmas runs on the real calendar, December 15 through the 31st, checked against your device clock rather than anything in the world. While it is active most enemies can drop Presents, several wear seasonal costumes, and hearts and mana stars are replaced by Candy Canes and Sugar Plums, which behave identically to the originals. A short list of items appears in NPC stock and on enemy drop tables for the season only, then disappears for eleven months.

Presents stack and open like any grab bag. Before Hardmode the contents are mostly novelty: seasonal food, decorations, and a couple of weapons like the Candy Cane Sword and the Fruitcake Chakram that are more fun than useful. Farm them the way you would farm any bag drop, with a spawn-boosted surface arena and something that clears the screen quickly, because the drop rate is low and the volume of kills is what carries it.

Hardmode is where they start to matter, because that is where the Snow Globe hides. What decides the roll is when you open the present, not when it dropped, so a stash left over from an earlier December is still worth carrying. Save them, beat the Wall of Flesh, then open the pile.

The Snow Globe summons the Frost Legion, and clearing that invasion is the gate on Santa Claus moving in. He needs an empty house like any other NPC, and he only takes it during the season, so the whole chain (reach Hardmode, farm presents, find a globe, beat the invasion, have housing ready) has to fit inside a two week real-world window or you are waiting a year for the next attempt.

He leaves when the season does. His stock is the seasonal outfit and the lights, garland and toppers that dress a Christmas Tree, so this is a completionist errand and a decorating one rather than a power spike. Worth doing early in the window rather than on the 30th.

Push the Frost Moon to wave 15 before dawn and the season turns on for the following in-game day whatever the real date is, which costs nothing extra if you were already farming that event for the Razorpine or the North Pole. A world made on the secret seed jingle all the way stays in Christmas permanently, which is the option if you want the drops on tap.

Failing both, moving your device clock into the window works the same way it does for the other seasonal event. Launchers that fake a date for one program spare you changing the whole system, and on console you disconnect from the network before setting the clock by hand.