The reward you can claim now is the 750 contribution winning side reward (10 adv refine).

If you got that, today is the chance to claim

If you don't, there is nothing to do. It will go away tomorrow.

It's done this way because this reward is for winning side only, which cannot be determined until the event period have ended and people have done "voting" with their orbs. So the reward have to be claimable AFTER the event ends.

I mean it would probably make everyone's life easier if it just mailed the reward to players but this is fine too.