Just give every player who completes the weekly challenge a token which can be used to unlock a piece of armor of choice, like a cataolge.
This way there is no FOMO because you dont miss out on specific armor pieces.
The thing is, people want to take a break, go on vacation for 1-3 weeks and miss out for 1-3 armor pieces, this is not player friendly.
Also, make the Event Challenges separate from the weekly challenges!!!