Some calendar watchers may have noticed a curious detail: July 25 did not mark the start of the seventh month on the Chinese calendar, but rather the Leap Sixth Month.

The Chinese calendar is a traditional lunisolar system, designed to reflect both the moon's phases and the Earth's seasonal cycles. 

Each month follows the synodic month, the period between two new moons, averaging about 29.53 days. A standard Chinese calendar year includes 12 of these months, totaling 354 or 355 days, roughly 11 days shorter than the solar year of 365.24 days.

Over time, this gap causes the months to drift out of sync with the seasons. To correct this, ancient Chinese astronomers developed a leap month system — by adding seven extra months every 19 years, the calendar realigns itself with the solar year.

The addition of a leap month depends on the "24 solar terms", which divide the sun's annual path into 24 equal parts. These terms mark seasonal changes and guide farming activities. 

Each month should contain at least one major solar term (zhong qi), such as the Start of Spring or Autumn Equinox, which falls around the middle of each month. Minor solar terms (jie qi), such as Start of Autumn or Grain Rain, appear earlier in the month and provide further seasonal cues.

If a lunar month contains only a minor term and no major term, it is designated as a leap month, repeating the name of the preceding month.

This is what occurred this year: July 22, or the 28th day of the sixth month on the Chinese calendar, coincided with Major Heat, one of the 24 solar terms. However, the following month, from July 25 to August 22, contains only Start of Autumn, a minor term, with no major solar term. As a result, this month is designated as a Leap Sixth Month, lasting 29 days.

Although unusual, this is not a once-in-a-lifetime event. The Chinese calendar will feature a leap sixth month six times this century: 2017, 2025, 2036, 2055, 2074, and 2093.

Source: Official WeChat account of General Office of the Shanghai Municipal People's Government at "shanghaifabu"