Ramadan follows the lunar (Hijri) calendar, which is based on the sighting of the moon. Because of this, Ramadan moves earlier each year in the Gregorian calendar and may begin on slightly different dates depending on moon sighting in each region.

The Prophet ﷺ said: "Start fasting on seeing the crescent and give up fasting on seeing the crescent. If the sky is overcast, then complete thirty days of Sha'ban." - Sahih al-Bukhari