The Hijri calendar is so-called after the migration, or hijra, of the Prophet Mohammed and the other early Muslims to Medina in the year 622 AD to escape persecution in Mecca.
The Hijri year is based on lunar cycles. It is made up of twelve months beginning with Muharram, and has 354 days, which corresponds to the time it takes for the moon to circle the earth twelve times.
The Hijri calendar is not aligned with the widely-used Gregorian calendar, which is calculated based on the earth's orbit around the sun and hence has 365 days a year and 366 on 'leap years'.
As a result, as you will see in our infographic below, the Hijri year 'loses' around 11 days a year to the Gregorian calendar.
This is why the Hijri New Year does not start at the same time every year relative to the Gregorian calendar.