The dates of Ramadan change because the Islamic calendar – also known as the Hijri calendar – is lunar, following the moon's phases. Hijri years are around 11 days shorter than Gregorian years, and this means the start of Ramadan moves forward by 11 days every year. The dates of the start and end of Ramadan can also vary slightly because they depend on sighting the new crescent moon – depending on when this happens, Ramadan might start or finish a day or two earlier or later than expected.