luni-solar calendars

Luni-solar calendars track days and months using the Moon's phases, like a lunar calendar, but periodically adjust to stay in step with the solar year and its seasons.

• Since 12 lunar months add up to only 354 days — about 11 days short of the solar year — the gap builds up to nearly a full month every 2–3 years.
• To fix this, an extra intercalary month (Adhika Maasa) is inserted every few years, realigning the lunar months with the solar seasons.
• Because of this correction, festivals that follow luni-solar calendars — unlike purely lunar ones — only shift by less than a month across the Gregorian calendar, rather than drifting through every month of the year.
• Many traditional Indian calendars, with month names like Chaitra and Vaisakha, are luni-solar — some counted as Amant (month ends on new Moon) and others as Purnimant (month ends on full Moon).