Obon was a lunar festival until Japan switched to the Gregorian calendar in 1873. Some regions kept the old date, some took the new one, and some split the difference.

Tokyo largely observes it in July, most of the rest of the country in August, and a minority still calculate the original seventh lunar month. It is a clean natural experiment in what happens when a calendar changes underneath a festival: the date fractures, and everybody insists their version is the traditional one.