Lunar Calendar is a calendar based on the monthly cycles of the Moon’s phases (synodic months). Lunar months are brought into alignment with the solar year through some process of intercalation.
Each lunation is approximately 29.5 days, lunar calendar to alternate between 29 (hollow) and 30 days (full). Since the period of 12 such lunations, a lunar year, is only 354 days, purely lunar calendars lose around 11 days per year relative to the Gregorian calendar. The most common form of intercalation is to add an additional month every second or third year.
Although the Gregorian calendar is in common and legal use in most countries, Lunar Calendar continue to be used throughout the world to determine religious festivals and national holidays. Examples of such holidays include: Chinese New Year and Mid-Autumn Festival (Chinese calendars).