The Islamic calendar is based on lunar months, which begin when a thin new crescent Moon is actually sighted in the western sky after sunset within a day or so after the New Moon. Hence, the month is either 29 days or 30 days.
There are 12 months in an Islamic year, which is either 354 days long or 355 days long, compared to (Gregorian) civil calendar year of 365 or 366 days. Since the Islamic Lunar year has 12 lunar months, it is on an average, 11 days shorter than the (Gregorian) civil year, the Islamic year shifts earlier in each civil year by about 11 days.
The 12 months of the Islamic calendar are: