The Dates of the Seventh Lunar Month Change Every Year: Why and When It Falls in This Year

Two Time Scales, Two Starting Points

The Gregorian calendar we use daily is a solar calendar (yangli), based on the Earth's orbit around the Sun, with one year lasting approximately 365.2422 days. The lunar calendar (nongli), however, is a lunisolar calendar (yinyang heli), where months follow the moon's phases, using the moment of "new moon" (shuo, when the moon is completely invisible) as the first day of each month. The lengths of these two scales are inherently different, so the misalignment of dates is structural, not a miscalculation by either system.

The Length of a Lunar Month: 29 or 30 Days

A synodic month—the period from one new moon to the next—averages about 29.53 days. The lunar calendar has no fractional days, so each month must be either 29 days (a "short month") or 30 days (a "long month"). The specific arrangement is determined by precise astronomical calculations, not a fixed alternation of short and long months.

Twelve synodic months add up to about 354 days. Compared to the 365.2422 days of a tropical year, this is roughly 11 days short. This 11-day difference accumulates each year, which, on the Gregorian calendar, means every lunar date appears approximately 11 days earlier than in the previous year.

Leap Months "Push the Dates Back"

If it were only 11 days short each year, the seventh lunar month would drift from midsummer to early spring within a few years, causing the calendar to lose sync with the seasons. To solve this, the lunar calendar has a leap month (runyue) mechanism: approximately every two to three years, an extra month is inserted at a certain point, making that year 13 months long. In the year a leap month is inserted, the dates jump forward, roughly compensating for the accumulated lag.

To summarize the rule in one sentence: Lunar dates "move back about 11 days each year on the Gregorian calendar, and jump forward about 19 days when a leap month occurs." This oscillation follows a discernible pattern, not random jumps.

Why Solar Terms Barely Move

Solar terms (jieqi) operate on entirely different logic. They are defined by the sun's position on the ecliptic (one term every 15 degrees), essentially a solar concept, so their dates on the Gregorian calendar only fluctuate by a day or two each year. This is why solar terms like "Liqiu" (Start of Autumn) and "Chushu" (End of Heat) almost always appear around the same Gregorian dates, while the dates of the seventh lunar month can span several Gregorian months.

The Seventh Lunar Month in 2026

For example, in 2026: the first day of the seventh lunar month is Gregorian August 13, and the 15th day (Zhongyuan Festival) is August 27. If you check the previous or following year, you will see different Gregorian dates; that is the result of the aforementioned 11-day backward shift or the leap month's forward jump. Our website's almanac page provides a day-by-day comparison of the lunar and Gregorian calendars, and the twenty-four solar terms table lists the precise Gregorian moments for each term.