Japanese Traditional Calendar (Kyureki)

Japan adopted Chinese lunisolar calendrical science as a matter of state administration beginning in the late seventh century, with the Gembu calendar introduced via the kingdom of Baekje in 690 CE, and over the following centuries the imperial court successively adopted a run of further Chinese systems, Genka, Giho and, from 862, the Senmyo calendar, which remained officially unrevised for more than eight hundred years, an unusually long span attributable as much to institutional inertia as to the calendar's own accuracy. The first calendar computed by a Japanese astronomer rather than imported from China arrived only in 1684, when Shibukawa Shunkai's Jokyo calendar replaced the by then badly drifted Senmyo system, and further Japanese-computed reforms followed, Horyaku and Kansei among them, ending with the Tenpo calendar of 1844, the last of Japan's traditional lunisolar calendars and the one still meant by kyureki, "the old calendar," today. All of these systems shared the same lunisolar mechanics inherited ultimately from China: twelve lunar months with an intercalary thirteenth added periodically to keep the count aligned with the solar year, months reckoned by the moon's phase rather than a fixed day count.

The Meiji government ended the traditional calendar's official status by decree in 1872, announcing with only a month's notice that the second day of the twelfth month of Meiji 5 would be followed immediately by the first of January 1873 on the Gregorian calendar, a change driven by the practical need to align Japan's administrative and diplomatic calendar with the Western powers it was rapidly opening relations with. Kyureki did not disappear from religious and festival life once it lost its civil authority, though.

Many traditional observances continue to be kept by their old lunar dates rather than by the fixed Gregorian date assigned to them after the reform, most visibly Obon, the midsummer festival honoring ancestral spirits, which some regions and shrines still keep by the seventh lunar month, kyu-bon, rather than by the fixed August or July Gregorian dates most of urban Japan now uses. Fishing and farming communities in particular have continued consulting the old lunar dates for tidal and seasonal purposes long after 1873, and annual almanacs still publish the kyureki dates alongside the Gregorian calendar specifically so festivals, planting and tidal work tied to the older reckoning can still be tracked. The calendar is retired from government use but not from the religious and practical life that continues to consult it.