What are the most important differences between the Muslim and Gregorian calendars?

The Muslim calendar, also known as the Islamic or Hijri calendar, is based on the lunar cycle and has 12 months in a year of 354 or 355 days. It is primarily used for religious purposes. The Gregorian calendar, which is the most widely used calendar in the world, is a solar calendar with 12 months in a year of 365 or 366 days. It is a standard calendar for civil purposes.