The mayor lost the keys to his safe. He asked you to find the keys and open the safe. During this event, at regular levels and in Challenge mode, you can collect Green Keys. If you collect the required number of Keys within the allotted time, you'll get a big reward; if you don't manage it in time, you'll get a consolation prize.
The time of the event is based on the time on your PC, and the duration may vary slightly from the duration of this announcement.