intercalary month; leap month
A month inserted into a lunar calendar to keep it aligned with the solar year. Used in traditional East Asian calendars.
In the old lunar calendar, an intercalary month is inserted about once every three years.
This year has a leap month, so the seasonal drift is adjusted.
Standard kanji form for the lunar calendar intercalary month.
閏 (intercalary) + 月 (month). The reading うるうづき is a native Japanese reading (kun'yomi) of the kanji compound, while じゅんげつ is the Sino-Japanese reading (on'yomi).