To prevent the lunar calendar from becoming more than half a month of sync with the solar calendar, an extra "leap month" is added in the Chinese calendar every 32 or 33 months. So every second or third Chinese calendar year has 13 months and 383–385 days. The last Chinese calendar leap month was the second lunar month 2 in 2023.

These calendars were based on the cycles of the moon, which average approximately 29.5 days. As a result, lunar calendars naturally align with a 13-month system, with each month consisting of approximately 28 days.

When dividing an annual cycle in lunar cycles, we get: 365 days / 28 days = 13 moons + 1 day In other words, the 13 Moon calendar is based on 13 moons of 28 even days per solar cycle + 1 day of transition (13×28 = 364+1). The 28 days per moon are divided in 4 weeks of 7 days each, giving 52 even weeks per year.