Animal Crossing: New Horizons' International Museum Day begins on May 18 and ends on May 31. Players will be able to participate in a Stamp Rally, which consists of finding the locations of stamp stations around the museum and earning a special reward.
Players should start the Stamp Rally by approaching Blathers who will talk about International Museum Day and Stamp Rally. After speaking to Blathers, you can discover stamp stations across the museum. Press A at these stations and complete the task by marking you card with a stamp.
Stamp Rally ACNH: How to Complete
There will be three different stamps in every exhibit, and after players have filled out their Stamp Cards, you can receive a prize depending on which exhibit you've visited and collected stamps in.
You should talk with Blathers again in order to complete the Stamp Rally. Blather will then hand your reward, which could be a bug, fish, art, or fossil plaque. If you've completed all Stamp Cards for the day, you will have also finished every activity for International Museum Day. Players can repeat these tasks the next day, but they will receive the same plaques for finishing them.
Although you will receive the same plaques, players will notice that stamp stations will be in different locations on certain days. Read more about finding these locations here.