INTERCALATION

insertion of days into a calendar to bring it into line with the solar year (year of the seasons). One example is the periodic inclusion of leap-year day (February 29) in the Gregorian calendar now in general use. To keep the months of a lunar calendar (e.g., the Hindu calendar) in their proper seasons, an entire month must be intercalated or extracalated (dropped) periodically, because there are a fractional number (between 12 and 13) of cycles of lunar phases (months) in a solar year. In astronomically primitive cultures, intercalation and extracalation were done empirically, whenever seasons and their properly associated months became noticeably out of step.