Allah Almighty has said,
"Surely the number of months with Allah is twelve in Allah's ordinance since the day He created the heavens and the earth, of these four are sacred; that is the right reckoning; therefore, do not be unjust to your own selves regarding them (Holy Qur'an, Surah Tawbah 9:36)."
These are the lunar months upon the reckoning of which does a Muslim in the east of the earth or the west rely; chronologically arranged, they are as follows:
3) Rabi' I,
4) Rabi' II,
5) Jumada I,
6) Jumada II,
9) The month of Ramadhan,
11) Thul-Qi'da, and
According to astronomy, the lunar calendar cannot be less than 29 days, nor can it be more than 30. It may once be 29 days and another 30, and its average is 29 days and 12 hours and five minutes. The beginning of each lunar month is recognized by the sighting of the new moon, the crescent. The Almighty says,
"They ask you concerning the new moons. Say: They are times appointed for the benefit of men, and for the pilgrimage" (Holy Qur'an, Surah Baqarah2:189).
In this verse, the Almighty has explained to us how to calculate and determine time by mentioning the word ahilla, which is the plural of the Arabic singular hilal, crescent, when it becomes visible to the naked eye. These crescents set the time for people and help them determine when the pilgrimage is to be performed.