So, welcome to the game it is really enjoyable on the climb so I’m glad you are experiencing this
If you’re saving coins for pulls here are my thoughts (which I think most people will agree with):
Most valuable challenge event to pull: Guardians of Teltoc
Atlantis: Pull with coins when there is a featured hero/HOTM that you have better odds of getting (for example next month Kingston is featured who is an excellent nature hero)
Valhalla: Since there is a trickle-down introduction of heroes it may behoove you to wait until the majority or all the Valhalla heroes are released so you can get the optimal hero. This would mean sitting on Valhalla coins which I can understand can be frustrating.
EHT: You’re right that Christmas currently would be your best bang for the buck, you would just want to make sure you are aware of the HotMs for December and January and decide to pull the tokens for the more favorable one to increase your odds there.
ToL: Pull whenever, these are the worst summoning odds in the game and are barely worth mentioning. If you want to “wait” until a hero is added or featured go ahead but there’s really no point.
Costume Keys: These are good to pull whenever there is a featured hero you would like. There are still a batch of Season 1 heroes pending their costume (Marjana comes to mind) so if you want one that’s available now, pulling sooner rather than later means you won’t have to deal with a further diluted summoning pool
Edit: I also changed your title as I was going to flag it because the title sounded like you were just sharing what you summoned during current challenge event.