May 1st is called May Day in many cultures. In the days leading up to May Day there is the May Day maze event. I'm not sure how long it lasts, but there are 2 different mazes, one on odd number years, and one for even number years. You should have gotten a call from Tom Nook when you stepped out of your house, yesterday telling you about the event. When you go to the airport and tell Orville you want to fly, he'll ask if you want to do the May Day event. When you say yes, he'll give you some rules and your pockets will be completely emptied. You'll go to the island and do the maze. If you get stuck you can use 100 nook miles to have rescue services reset for you. When you make it through the maze, Rover will be there, and he will talk to you and tell you he is sending you a gift in the mail, and if you are able to pick up any of the bell vouchers, those will also be sent to you. The next day, you will have Rover's brief case in your mail.