How is the beginning of the lunar month determined in the Hijri calendar?

The beginning of the lunar month in the Hijri calendar is established by sighting the new crescent moon. The sighting takes place at sunset on the 29th day of the preceding month. If the crescent is sighted, the new month begins; if not, the previous month must be completed as thirty days. This is based on the words of the Prophet (peace and blessings be upon him) regarding the month of Ramadan, “Fast when you see it [the new crescent] and end your fast when you see it. If it is obscured from you, then complete thirty days of Sha‘ban” (recorded by Bukhari and Muslim).

The principle for establishing the beginning of the lunar month is to rely on physically sighting of the crescent, whereas astronomical calculations serve only as a supporting tool. The adopted view for fatwa is that astronomical calculations can be used to rule out, but not to establish, the sighting of the crescent. Therefore, if the calculations show that the crescent cannot be sighted, no consideration is given to any claim of sighting. Confirmation of the crescent’s appearance is accepted only by visual sighting, provided astronomical calculations do not preclude the possibility of such a sighting.

And Allah Almighty knows best.