Fast when you see it, and break the fast when you see it, but if the sky is overcast and you cannot see it, then act on estimation

Explanation

The Prophet (may Allah's peace and blessings be upon him) pointed out the sign that marks the beginning and end of the month of Ramadan, saying: Fast when you see the crescent of Ramadan, but if the clouds prevent you from seeing it, then count thirty days for the month of Sha‘bān. Break your fast when you see the crescent of Shawwāl, but if the clouds prevent you from seeing it, then count thirty days for the month of Ramadan.

Benefits

• Sighting the crescent, not the calculations, is what counts in affirming the beginning of the lunar month.
• Ibn al-Mundhir reported scholarly consensus that fasting is not obligatory if the beginning of the month of Ramadan is determined only by calculations without sighting the crescent.
• It is obligatory to complete Sha‘bān thirty days in case the crescent of Ramadan cannot be seen due to clouds or the like.
• The lunar month is either twenty-nine or thirty days.
• It is obligatory to complete Ramadan for thirty days in case the crescent of Shawwāl cannot be seen due to clouds or the like.
• Whoever finds himself in a place where there is none to check on Muslims' affairs in fasting, or he does not pay attention to this, then he should be careful about it and should seek it through someone who affirms it either by sighting it himself or by sighting it by one whom he trusts, and he can thus fast and break his fast accordingly.