The Islamic calendar has 12 months with 29 or 30 days. If the Crescent Moon is visible shortly after sunset on the evening of day 29, the following day is the first day of the new month. If no sighting is made, a 30th day is added to the current month, which is then followed by the first day of the subsequent month.
Some countries and Muslim communities now use modified versions of the traditional calendar that are designed to make the timing of Islamic months and observances easier to predict.
A new month may also begin on different days in different countries. Because the time of moonset at a location depends on its longitude, a new month and key religious rituals like the Ramadan fast may begin a day earlier in, for example, West African Muslim countries than in Indonesia or Malaysia.
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Source: Al Habib Islamic Calendar