Bismillah.
The Islamic Calendar - Astronomy
In the Islamic calendar, each month has either 29 or 30 days, but usually in no discernible order. Traditionally, the first day of each month is the day (beginning at sunset) of the first sighting of the hilal (crescent moon) shortly after sunset. If the hilal is not observed immediately after the 29th day of a month (either because clouds block its view or because the western sky is still too bright when the moon sets), then the day that begins at that sunset is the 30th. Such a sighting has to be made by one or more trustworthy men testifying before a committee of Muslim leaders. [Wikipedia]
Volume 3, Book 31, Number 130:
Narrated Abdullah bin Umar (radiallaahu `anhu):
Allah's Apostle (sallallaahu `alayhi wasallam) mentioned Ramadan and said, "Do not fast unless you see the crescent (of Ramadan), and do not give up fasting till you see the crescent (of Shawwal), but if the sky is overcast (if you cannot see it), then act on estimation (i.e. count Sha'ban as 30 days)."
Volume 3, Book 31, Number 131:
Narrated Abdullah bin Umar (radiallaahu `anhu):
Allah's Apostle (sallallaahu `alayhi wasallam) said, "The month (can be) 29 nights (i.e. days), and do not fast till you see the moon, and if the sky is overcast, then complete Sha'ban as thirty days."
Volume 3, Book 31, Number 132:
Narrated Ibn 'Umar (radiallaahu `anhu):
The Prophet (sallallaahu `alayhi wasallam) said, "The month is like this and this," (at the same time he showed the fingers of both his hands thrice) and left out one thumb on the third time.
Volume 3, Book 31, Number 133:
Narrated Abu Huraira (radiallaahu `anhu):
The Prophet (sallallaahu `alayhi wasallam) or Abu-l-Qasim said, "Start fasting on seeing the crescent (of Ramadan), and give up fasting on seeing the crescent (of Shawwal), and if the sky is overcast (and you cannot see it), complete thirty days of Sha'ban."
Volume 3, Book 31, Number 134:
Narrated Um Salama (radiallaahu `anhu):
The Prophet (sallallaahu `alayhi wasallam) vowed to keep aloof from his wives for a period of one month, and after the completion of 29 days he went either in the morning or in the afternoon to his wives. Someone said to him "You vowed that you would not go to your wives for one month." He replied, "The month is of 29 days."
Volume 3, Book 31, Number 135:
Narrated Anas (radiallaahu `anhu):
Allah's Apostle (sallallaahu `alayhi wasallam) vowed to keep aloof from his wives for one month, and he had dislocation of his leg. So, he stayed in a Mashruba for 29 nights and then came down. Some people said, "O Allah's Apostle (sallallaahu `alayhi wasallam)! You vowed to stay aloof for one month," He replied, "The month is of 29 days."
Volume 3, Book 31, Number 136:
Narrated Abu Bakra (radiallaahu `anhu):
The Prophet (sallallaahu `alayhi wasallam) said, "The two months of 'Id i.e. Ramadan and Dhul-Hijja, do not decrease (in superiority)."
This has been interpreted in two ways by the scholars:
- the two months of Eid (Ramadan and Dhul Hijjah) do not decrease in number (in the same year). For example, if this year, Ramadan is 29 days, then Dhul Hijjah will definitely be 30. And if Dhul Hijjah is 29, then Ramadan will be 30. Both cannot be 29 simultaneously.
This does not have to be the case each time. This is why the second opinion is preferred and more correct. Allah knows best. - these two months do not decrease in superiority. Even if the number decreases to 29, the reward of the people who do good in these two months will not be decreased. :)
Volume 3, Book 31, Number 137:
Narrated Ibn 'Umar (radiallaahu `anhu):
The Prophet (sallallaahu `alayhi wasallam) said, "We are an illiterate nation; we neither write, nor know accounts. The month is like this and this, i.e. sometimes of 29 days and sometimes of thirty days."
