Started in 1977, International Museum Day “encourages public awareness of the role of museums in the development of society”.
To celebrate, many museums in Tokyo—including the Tokyo National Museum, the National Museum of Nature and Science, the National Museum of Western Art and the Shitamachi Museum (all in Ueno) will have free admission.
This is no secret though, so be prepared for large cheapo crowds.
Please note that International Museum Day is “on or around the 18th May” so check with individual museum’s websites to ensure free entry.
Organizers may cancel events, alter schedules, or change admission requirements without notice. Always check official sites before heading to an event.