Islam dates from the Hijra, or migration from Mecca to Medina. This is year 1, AH (Anno Hegira)—which
corresponds to AD 622 or 622 CE, depending on the notation preferred (see Common era). It is a lunar calendar, but differs from other such calendars (e.g. the Celtic calendar) in that it omits intercalary months, being synchronized only with lunation s, but not with the solar year, resulting in years of either 354 or 355 days. This omission was introduced by Muhammad because the
right to announce intercalary months had led to political power struggles. Therefore, Islamic dates cannot be converted to
the usual CE/AD dates simply by adding 622 years. Islamic holy days fall on fixed dates of the lunar calendar, which means
that they occur in different seasons in different years in the Gregorian calendar.