Allah says in the Qur'an:
"The number of months in the sight of Allah is twelve (in a year) - so ordained by Him the day He created the heavens and the earth...." (At-Tawbah(9):36).
"It is He Who made the sun to be a shining glory, and the moon to be a light of beauty, and measured out stages for it, that you might know the number of years and the count of time. Allah did not create this except in truth and righteousness. And He explains His signs in detail, for those who understand" (Yunus(10):5).
And in his final sermon before his death, the Prophet Muhammad(peace be upon him) said, among other things,
"With Allah the months are twelve; four of them are holy; three of these are successive and one occurs singly between the months of Jumaada and Sha'ban."
Islamic months begin at sunset of the first day, the day when the lunar crescent is visually sighted. The lunar year is approximately 354 days long, so the months rotate backward through the seasons and are not fixed to the Gregorian calendar.
(The months in Hijrah year)
The months of the Islamic year are:
2. Safar ("Empty" or "Yellow")
3. Rabia Awal ("First spring")
4. Rabia Thani ("Second spring")
5. Jumaada Awal ("First freeze")
6. Jumaada Thani ("Second freeze")
7. Rajab ("To respect" - this is another holy month when fighting is prohibited)
8. Sha'ban ("To spread and distribute")
9. Ramadan ("Parched thirst" - this is the month of daytime fasting)
10. Shawwal ("To be light and vigorous")
11. Dhul-Qi'dah ("The month of rest" - another month when no warfare or fighting is allowed)
12. Dhul-Hijjah ("The month of Hajj" - this is the month of the annual pilgrimage to Mecca, again when no warfare or fighting is allowed)