All prefectures and wards in Japan have a “Residents Day”.
Wards (similar to U.S. counties) normally have a festival for their Residents Day … and prefectures (similar to U.S. states) usually give kids the day off from school and that prefecture’s zoos and museums offer free admission.
Today (October 1st) is 都民の日 (Tokyo Residents Day).
Normally zoos and museums in Japan are closed on Mondays.
Today is a Monday, but Tokyo’s two government-owned zoos (Ueno Zoo and Tama Zoo) are open today and admission is free.
The zoos in Japan always on Mondays if it’s a holiday (though free only on a few days a year)…but they close the day after the holiday — so Tokyo’s zoos will be closed tomorrow (October 2nd).
A typhoon hit the Tokyo area yesterday but the weather is nice today, so the zoos are sure to be crowded today.
So if you’re in Tokyo now, and you don’t mind a crowd, you can go to one of Tokyo’s zoos for free today (I recommend Tama Zoo but it’s not as conveniently located as Ueno Zoo).
Just don’t plan to go to a zoo or museum in Tokyo tomorrow … they’ll be closed (the ones in the neighboring prefectures are closed today but will be open tomorrow … as usual).