The Islamic calendar is a lunar calendar of 12 lunar months (giving a year of 354 or 355 days). It is used in Muslim countries around the world (in addition to the Gregorian calendar), internationally by Muslims, and sets the dates for Islamic holy days and festivals.
The first year of the Islamic calendar is set by the migration of the Islamic prophet Muhammad from Mecca to Medina (17 July 622 CE). The Islamic year is denoted by the letter H, for Hijra, or AH, for anno Hegira (in the year of the Hegire).
There are twelve Islamic calendar months:
Muharram, meaning "forbidden" -- it was forbidden to fight during this month. Considered the second most sacred Muslim month
Safar, meaning "void" or "empty" -- either because people would leave Mecca during this month, or because they would use the month to raid other tribes and leave them with nothing (empty)
Rabīa al-Awwal, meaning "first spring"
Rabīa ath-Thānī, meaning "second (or last) spring"
Jumādā al-Ūlā, meaning "first dry month, or freeze"
Jumādā ath-Thāniya, meaning "second (or last) dry month, or freeze"
Rajab, meaning "honor or respect" -- it was also forbidden to fight during this month.
Sha'bān, meaning to scatter or distribute" -- the month when tribes would separate to search for water
Ramadān, meaning "parched" -- this is the month of fasting between dawn and sunset, the most important month of the Islamic calendar
Shawwāl, meaning "upraised and vigorous" -- the usual month of birth of camel stock amongst the tribes
Dhū al-Qa'dah, meaning "the month of truce" -- another month when warfare was forbidden between the tribes
Dhū al-Hijra, meaning "the month of pilgrimage" to Mecca.