So basically you know your friend Shadow, having new ideas every 4 seconds. Here's another one I came up with!
Basically these Npcs will have difficult challenges for players to compete in a given time. Then if they can complete the challenge, players will get movements to a holiday specific spell, or some special items. This will spread more cheer around the server, and give players something challenging and fun to do when they might be bored. Here are some that I brainstormed.
Christmas
Npc - Elves
Quest: The Elves and Santa are behind on their Christmas morning delivery! The Elves will show up for 5 days, then which you have to find the children npcs hidden in the castle and deliver to them. You reward will be the Candy Cane Flare movements!
Valentine's Day
Npc - Cupid
Quest: Brew some more Love Potions! He will give you the steps and the ingredients to complete. This is only available on Valentine's day, and you can only brew the potions on Valentine's day. You will get 5 Amortentia splash potions. Amortentia potions cause all players in the splash radius to be bound together within three blocks for 5-7 seconds.
Easter
Npc - Bunny
Find the Eggs hidden around the castle! Available for 3 days. Reward is 32 confetti eggs. Smash them into the ground to release a whole bunch of particles that fly everywhere.
You get the idea