lunar calendars

Lunar calendars are calendars built entirely around the Moon's phases, using a day as the shortest unit and a month of about 29.5 days.

• A lunar year of 12 lunar months adds up to about 354 days — noticeably shorter than the roughly 365-day solar year that actually matches the seasons.
• Because of that 11-day gap, a purely lunar calendar's months drift steadily out of sync with the seasons, year after year, with no built-in correction.
• Despite this drift, lunar calendars were an early and genuinely reliable way to track time, since the Moon's phases are so visually obvious and repeat on a dependable schedule.
• Some religious festivals, like Eid-ul-Fitr, still follow a purely lunar calendar today — which is exactly why such festivals fall on a different Gregorian date, and even a different Gregorian month, from year to year.