They should make this permanent so the joker orbs are more useful. Started summoning a Legend last night and had to waste 40 gems to speed it up so I can get some of my missing dragons. Wish SP could annouce these events in advance so we can plan ahead to keep things cleared. Kind of like the breeding island where they annouce upcoming breedings.
Also, 24 hours is too short. Make it one week like the last 50 joker orbs summoning event. You have to burn through gems if you want to summon more than 1 dragon during this short window.
Then again, SP may be doing this intentionally to make us waste more gems.