Ramadan comes at a different time every year because it is the ninth month of the Islamic calendar, a date-keeping system based on lunar cycles, unlike the Gregorian calendar (the one used by most of the world, including the U.S.), which is based on the solar year. A new month begins with the appearance of the new moon, or the crescent moon, and ends with the next appearance of a new moon.

The month of Ramadan thus moves backwards about 10 days every year relative to the Gregorian calendar.

Different communities follow different protocols for determining when a new month begins. Some communities follow a set lunar calendar, others use scientific observations to make an official decree about the arrival of a new moon, and still, others mark a new month only after the actual sighting of the crescent moon in their community.

Though the exact dates of Ramadan are never uniform around the world, they come pretty close.