The calendar we use today is called the Gregorian calendar, named after Pope Gregory XIII. However, Islamic regions use a different calendar.
The Gregorian calendar, which we follow, keeps track of time using the sun and the moon. It's pretty accurate, with only one day off every 3,300 years. But the Islamic calendar is based on the movements of the crescent moon. That means their year is shorter than ours, with 354 days instead of 365.
Because of this difference, the Islamic calendar doesn't match up perfectly with ours. Imagine if your birthday is in the winter, but after a few years, it moves to the summer! To fix this, they have a special "leap year" 11 times in a 30-year cycle.
In the Islamic calendar, each month can be 29 or 30 days long. So some months are shorter, and some are longer.
