Establishing the beginning of the lunar month
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Question: How is the (beginning of the) month of Ramadhaan established?

Response: The beginning of the month of Ramadhaan is established either by the new moon being sighted, or by the passing of 30 days of (the month of) Sha'baan. This is based on the statement of the Prophet (sal-Allaahu `alayhe wa sallam):
((And if you see it (the new moon of Ramadhaan) then fast, and if you see it (the new moon of Shawwaal) then break you fast. And if the clouds block (you view) then complete the count of Sha'baan thirty (days))).

Shaykh Ibn 'Uthaymeen
Fataawa Ramadhaan - Volume 1, Page 56, Fatwa No. 21;
Fiqh al-'Ibaadaat - Page 172