International Museum Day is apparently a real annual event that takes place on May 18, and not being one to shy away from obscure holidays and seasonal events, Animal Crossing: New Horizons is treating the day as an in-game event. It's called the Stamp Rally, and it runs from May 18 until May 31. Like the May Day event that took place at the beginning of the month, the Stamp Rally gives you a week to visit your island’s museum in order to collect stamps and give you an excuse to spend some time fully appreciating Blathers’ beautiful building of bugs, bones, and bass (among other fish).