I imagine that many readers of this blog are familiar with Chinese New Year, which typically happens in February, and marks the start of the year on the Chinese lunar calendar. Until the late nineteenth century, Japan also used a lunar (strictly, a lunisolar) calendar, and originally it used the Chinese one. However, because those calendars were calculated for Chinese longitudes, they were not quite right for Japan, and they were replaced in the Edo period by specifically Japanese ones. The current Japanese lunisolar calendar is known as the Tenpō Calendar, because it was adopted in 1844, at the end of the Tenpō era.
The differences from the Chinese calendars are minimal, because both agree that months start on the day of a new moon, and are either 29 or 30 days long, depending on the month. Most years have twelve months, but an additional month is added seven times in nineteen years to keep the year roughly in sync with the seasons. This is based on the months in which particular events connected to the solar calendar occur.
The current Chinese and Japanese calendars do differ on the rules for additional months. For the Japanese Tenpō Calendar, the rule is that the Winter Solstice must always be in the eleventh month, the Spring Equinox must always be in the second month, the Summer Solstice must always be in the fifth month, and the Autumn Equinox must always be in the eighth month. The current Chinese calendar says that the Winter Solstice must always be in the eleventh month. In both cases, a month that does not include one of twelve fixed solar dates, including the solstices and equinoxes, is designated an additional month. On the Chinese calendar, this is the first one in a year with thirteen months between the solstices; on the Tenpō Calendar, it is chosen to keep the solstices and equinoxes in the right months. An additional month keeps the number of the month before it, so if it is added after the third month, it is called an additional third month.
Incidentally, this is probably part of the reason why birthdays are not traditional in this part of the world. People born on the 30th of any month would not have a birthday in about half the years, and people born in additional months would lose it most of the time. That’s a lot more people than those born on February 29th.
The lunisolar calendar is not officially used in Japan, but it is unofficially used in fortune telling (in deciding whether a day is Taian, and thus lucky, for example), and for fixing the day of some festivals. For example, there is a tradition of viewing the full moon in the eighth month of the old calendar.
There was an article about this in the April 18th issue of Jinja Shinpō, because there is a problem coming up.
In 2033, it will not be possible to follow the rules for the Tenpō Calendar. The timing of the phases of the moon means that it is impossible to have all of the solstices and equinoxes in their set months. This happens because the new moon is on the Autumn Equinox that year, and the Winter Solstice is one day before the third new moon after that. In other words, if the Autumn Equinox is on the first day of the eighth month, the Winter Solstice is on the last day of the tenth, and to have the Winter Solstice in the eleventh month, you must have the Autumn Equinox in the ninth.
There are three possible months for the additional month; an additional seventh month, an additional eleventh month, or an additional first month in the following lunar year. An additional seventh month keeps the Autumn Equinox in the eighth month, while an additional eleventh month keeps the Winter Solstice in the eleventh month. (I think that an additional eleventh month is what the Chinese calendar will do, so that option would also be a way to keep the Chinese New Year and old-style Japanese New Year in sync.)
The problem is that, because the government does not use the old calendar, there is no-one who has the authority to decide which one of these is right. The general tendency seems to be to prioritise the Winter Solstice and go for an additional eleventh month, but unless universal agreement is reached, people might publish different calendars for that year. Some jinja use the old calendar to determine their matsuri dates, for at least some matsuri (for example, the important Kamiarisai in Izumo — which would be affected), so the purpose of the article was to make priests aware of the potential problem, and get them involved in making sure that the Shinto world, at least, agrees on one solution.