Dates of the Islamic Calendar must tally with the phases of the moon.
An observer who has witnessed many new moons and full moons can easily identify the dates prescribed for man by Allah (swt) by watching the phases of the moon. This is clearly explained in simple language in the chapter of the Qur’aan, Yaseen and Yunus, which mention about the phases and Manazil of the Moon and their calculations. The rules of movements of the Sun and the Moon are subjected to computation (Q55: 5) and the dates can be predicted by calculations.
The waxing crescent becomes visible to the naked eye observer on the first day of the month after sun set.The last day of the month, moon has no phase and moon sets after the sun at some parts of the world, but the crescent is not visible. It becomes visible to earth only on the first day of the month after sun set.
The first day of the lunar month can be found out by monitoring the waning crescents in the last days of the month. The Quran describes the last visible phase as 'Urjoonul Qadeem'. This is seen one day before the end of the month.
If a month begins on Friday for example, next Friday will be the 8th. The first quarter is over by Thursday the 7th on which an observer sees the moon like a big crescent on the meridian at sun set. Thid confirms beginning of the month on Friday.
By next Friday the moon is opposite to the sun rising at the sun set. It is 15th Friday the full moon day. In months of 29 days setting of the sun and risnig of the moon will be almost simultaneous. In months of 30 days moon rises a few minutes before the sun set. Next morning i.e.16th, moon sets after the sun rise.
Next Friday 22nd of the month in the morning before sun rise moon is almost on the meridian like a half moon. In a month of 29 it will be almost haff. The next day Saturday morning it is slanting to the east looking like a big crescent.This indicates that next Saturday will be the first of the next month.
In months of 30 days it will be more than half and attains half on the next day. That is Saturday it is half and Sunday it is less than half looking like a big crescent. So Sunday will be the first of the next month.
Last observation is the urjoonul Qadeem. We must begin the observation 2 or 3 days before the end of the month. When urjoonul qadeem is seen it will be sure that next day it will not be seen even though the moon rises a few minutes befor the sun. This is the day of conjunction (last day) on which moon is invisible to earth.The next day is the first day of the new month and Hilaal becomes visible after sun set.
The Islamic months are based on the cycle of the Moon. The months begin with the appearance of the first Hilal. Now we shall examine whether the Imams and the Fuqaha (Jurists) agree to have common dates for the Islamic calendar, from the quotations from the writings of their own. Let us examine the views of the Imams of different Schools of Thought of the Muslim community regarding the observation of the Hilal to begin the months of the Islamic calendar: