All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
The day starts at dawn and not at sunset of the previous day, nonetheless the night which precedes the day follows it regarding rulings, for instance it is said the night of Friday when the sun sets on Thursday, and the night continues until dawn. Allaah Says about the Night of Decree (what means): {(All that night), there is Peace (and Goodness from Allaah to His believing Slaves) until the appearance of dawn.}[Quran 97:5]. The Taraaweeh prayers are performed when the crescent of the month of Ramadan has been sighted, and it is an obligation to have the intention of fasting from the night, and there are other rulings which apply, with the exception of the day of 'Arafah as the night which comes after it follows it regarding its rulings, as it is acceptable for a pilgrim to stand in 'Arafah during this night and his standing during the night is correct.
As regards the beginning of the month, it starts from the first night and not from dawn, according to the consensus of the scholars. Abu Bakr Al-Jassaas interpreted the saying of Allaah (which means): {…nor does the night overtake the day…}[Quran 36:40], saying: "No scholar has ever said that its meaning is that the beginning of the month is from the day, because interpreting it by this meaning is rejected. In addition to this, since the months which are related to Islamic rulings are lunar months, and the first time the crescent appears is by night and not at the beginning of the day, then the month must start from the night, and there is no difference of opinion among the scholars that the first night of Ramadan [the night which precedes the first day of Ramadan] is from the month of Ramadan, and that the first night of Shawwaal is from the month of Shawwaal. Therefore, it becomes evident that the month starts from the night, and they start Taraaweeh prayers from the first night [which precedes the first day] of Ramadan. It is reported that the Prophet said: "At the first night of Ramadan, the devils are chained." All the above is evidence that the beginning of the month is from the night. Indeed the scholars of our school [Hanafi] said about the people who say: 'I vow to make I'tikaaf to Allaah for one month', that they start from the night, because the beginning of the month starts from the night. Besides, Al-Badr Az-Zarkashi reported the interpretation of Al-'Izz Ibn 'Abdus-Salaam about the Saying of Allaah (which means): {It is not for the sun to overtake the moon nor does the night outstrip the day…}[Quran 36:40], who interpreted {It is not for the sun to overtake the moon} to mean in its 'kingdom', which is the night, i.e. the sun does not come during the night, and His Saying after this {…nor does the night outstrip the day. They all float, each in an orbit.}, means that the night does not come during the 'kingdom' of the sun which is the day. As regards the saying of the astronomers that the day starts from sunrise, if this is correct, their meaning is different from that of religion which has rulings about the starting and ending of the day.
For more details please refer to Fatwa 91097.
Allaah Knows best.