International Museum Day is an in-game event, lasting from May 18th to May 31st.
Enter the museum and talk to Blathers to start the event. This allows you to collect stamps at stations in the three main wings of the museum. There are no stations in the art gallery (if you've unlocked it).
When you've completed a stamp card, return to Blathers for your rewards:
And that's it - you've completed all that International Museum Day has to offer.
You can come back and complete each wing every day during the event. The stamp stations will move (there are many possible stations in each wing), but the rewards wont vary - just more plaques.
The three bug stations are randomly placed and vary each day. These are only examples.
Upon completion of the stamp card, Blathers rewards you with the Fish plaque .
The three fossil stations are randomly placed and vary each day. These are only examples.
Upon completion of the stamp card, Blathers rewards you with the fossils-plaque.
The three fish stations are randomly placed and vary each day. These are only examples.
Upon completion of the stamp card, Blathers rewards you with the Fish plaque .