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The dates of the ninth month of the Muslim calendar, Ramadan, vary from year to year, as the Muslim calendar (Hijrah) is based on a lunar cycle of 29 or 30 days. The exact date is determined by the sighting of the moon's position on the horizon. These lunar calculations lead to an official announcement by the Religious Affairs Ministry on the eve of Ramadan and Idul Fitri so that the faithful know when to begin and end the fasting month. The Muslim religious group Muhammadiyah uses a different, mathematical, calculation to determine the start/end dates, so many years the two official date announcements are actually different. Indonesian Muslims choose which one to follow.

In 2021, the fasting month of Ramadan will start on in mid-April and Lebaran will fall on May 13-14, with most offices and businesses taking "shared holiday/collective leave" from May 13 - May 19 (or longer)