The International Museum Day event is only available if at least one day has passed after the museum has been built or after the museum has been renovated to add the Art Gallery. During the event, the fossil, fish, and bug wings of the museum will each have three stamp stations, yellow carts with the museum logo holding stacks of cards and a stamp, placed at random locations within them. Speaking to Blathers will begin a Stamp Rally, in which the player must explore the wings and collect all their stamps. Collecting all three of a wing's stamps and speaking to Blathers again will reward a completion stamp and an exclusive plaque. There is one plaque for each wing: the fossil plaque, fish plaque, and bug plaque.
The player can participate in one Stamp Rally per day on each day of the event and obtain all three plaques multiple times. The stamp stations will be in different locations each day, but there are no additional rewards to obtain after the plaques.
Below is a list of exhibits showcased in the Stamp Rally:
In the main entrance room:
- Singing Forest, on the path directly west from the main entrance.
- "If you listen closely, you can hear a faint hum." (daytime)
- "It's so quiet at night..." (nighttime)
- Quiet Bench, the bench under a pair of palm trees on the west side.
- "Sometimes it's nice to take a short breather and relax on the bench."
- Sun-Peek Path, near where the jewel beetle and other tree stump bugs sit, in the center of the room.
- "There's something very relaxing about this space."
- Shady Path, directly north of the main entrance, just south of the large tree.
- "It seems like the temperate-climate bugs are relaxing here."
- Giant Tree, on the right side of the large tree in the northeastern area.
- "It seems the bugs gather for tree sap at night."
- Dragonfly Bridge, on the plank bridge in the north.
- "The reason you can see so many dragonflies near water is because that's where they lay their eggs!"
- Coconut Corner, near the palm trees in the northwestern area.
- "I bet tropical bugs get to go surfing and lay out on the beach all the time."
In the northern room:
- Hiding Bugs, in the east, by the set of three dioramas, and near the Orchid Mantis.
- "Bugs: Masters of disguise!"
- Bug Farms, in the back center of the room, near the largest terrarium.
- "It's like I'm part of a bug community. Hi, bugs!"
- The Lab, in the northwestern area, filled with cupboards and lab equipment.
- "I bet some groundbreaking research goes on here."
In the western room:
- Butterfly Garden, directly south of the fountain.
- "Sitting on a bench at this fountain... is about as classy as it gets."
Upon collecting all the stamps in the Bug Exhibit, Blathers will say:
"Your willingness to explore the bug rooms with such gusto is...admirable. I, myself, find lingering there quite impossible!"
In the main entrance room:
- Serenity Tank, near the tank shaped like a figure 8 near the entrance, with goldfish and other small fish.
- "Little guys, just chillin' in a little tank. Must be nice."
- The Pond, the topless tank in the center of the room, filled with lily pads and tall grass.
- "I feel like these guys are little aquatic athletes. Go, team!"
- Headwaters, near the tank in the north, with a three-tiered river.
- "Different fish live in different parts of the same river. Wow!"
- Big River, the large tank at the far right of the room.
- "Some rivers are deep, and wide, and looooooooooooooong..."
In the northern room:
- The Coast, a large tank in the western side, at the bottom of the stairs.
- Coral Reef, at the top of the stairs, in an open area near the tank with the Clownfish.
- "Oooh, a colorful, vibrant, warm sea. I want to swim here."
- Icy Sea, in the top right corner of the room, near the Sea Butterfly.
- "That's as chilly as it gets!"
In the eastern room:
- The Deep, in the center of the large open area by the large tank.
- "That is one big ocean!"
- Abyss, a tank containing deep-sea fish like the Oarfish and Coelacanth.
- "How far down are we?"
Upon collecting all the stamps in the Aquarium, Blathers will say:
"I say, did you happen to take any photographs of your quest? Your NookPhone takes marvelous photos. And the fish rooms make an elegant backdrop, to be sure."
In the first room:
- Mollusks/Arthropods, on the left.
- "No backbone. But still tough, I bet."
- Vertebrates, on the right.
- "Now THESE guys have some backbone..."
In the second room:
- Aquatic Reptiles, right near the entrance, by the Plesiosaurus.
- "They should call this display "NOT Dinosaurs, Chapter One.""
- Pterosaurs, north of the entrance, near the Quetzalcoatlus.
- "They should call this display "NOT Dinosaurs, Chapter Two.""
- Extinction Spot, by the blue circle in the center of the room, to the right of the Diplodocus.
- "What's that big orb near the ceiling? I'm sure it's fine..."
- Synapsids, to the right of the display closest to the meteor sculpture.
- ""Distant relatives of mammals." Huh."
In the third room:
- Cenozoic, in the center of room.
- "What are those silhouettes above and behind them?"
Upon collecting all the stamps in the Fossil Gallery, Blathers will say:
"As the host of these rallies, it pleases me to my pinfeathers to see you frolic amid the fossils so!"