Many ancient calendars like Hebrew, Chinese and Buddhist calendars had entire leap months since there was a natural gap of roughly 11 days between a year as measured by lunar cycles.
Many calendars, including the Hebrew, Chinese and Buddhist calendars, are lunisolar, meaning their dates indicate the position of the Moon as well as the position of Earth relative to the sun. Since there is a natural gap of roughly 11 days between a year as measured by lunar cycles and one measured by the Earth's orbit, such calendars periodically require the addition of extra months, known as intercalary or interstitial months, to keep them on track.
Intercalary months, however, were not necessarily regular. Historians are still unclear as to how the early Romans kept track of their years, mostly because the Romans themselves may not have been entirely sure. It appears that the early Roman calendar consisted of ten months plus an ill-defined winter period, the varying length of which caused the calendar to become unpegged from the solar year.
Eventually, this uncertain stretch of time was replaced by the new months of January and February, but the situation remained complicated. They employed a 23-day intercalary month known as Mercedonius to account for the difference between their year and the solar year, inserting it not between months but within the month of February for reasons that may have been related to lunar cycles.