the hegira the name given to the emigration of the Prophet and his fllowers from mecca to medina
In this way a new way of counting the time that constitutes the Islamic calendar was established. In reality, this system comes from the old calculation established by the pre-Islamic Arabs, consisting then of a lunisolar calendar with a year of twelve months that included, alternately, 29 and 30 days
so know a days because of these the muslim people have a different calendar
This system was readapted by the Prophet during the pilgrimage of AH 10, in which he forever prohibited (Qur'an, 9, 36-37) the intercalations of months.