Obon is kept on different dates in different parts of Japan, and the split has a precise cause: the adoption of the Gregorian calendar on 1 January 1873.
Before that, Obon fell on the fifteenth day of the seventh lunar month, late summer, after the heat had peaked, at full moon. The reform forced a choice, because the number and the season could no longer both be kept.
- Shichigatsu Bon, July Bon, kept the number. Regions that took this route observe July 13-15 in the new calendar. Tokyo, Yokohama and much of the Kanto and Tohoku do this.
- Hachigatsu Bon, August Bon, kept the season. Shifting a month forward to August 13-15 put the festival back roughly where the old seventh month had sat. Most of the country took this option, which is why the national travel peak and the corporate shutdown fall in mid-August.
- Kyu Bon, old Bon, kept the calendar. A minority of areas, parts of Okinawa, the Amami islands, and some rural districts, still calculate the fifteenth day of the seventh lunar month, so their Obon moves every year and can fall anywhere from late August into September.
The consequence is that Japan has no single national Obon date, only a dominant one, and that the answer to when Obon is depends on which prefecture is asking.
The story underneath
The framing narrative is the Ullambana Sutra: Maudgalyayana, greatest of the Buddha's disciples in psychic power, looks for his dead mother and finds her among the hungry ghosts, unable to eat. He is told he cannot free her alone and must make offerings to the whole assembly of monks at the close of the rains retreat. The sutra is generally judged by scholars to be a Chinese composition rather than a translation from an Indian original, shaped by Chinese concern with filial duty. Beneath the Buddhist frame there is older Japanese practice around the returning dead, and the festival is best read as the two settled onto one another.