The start date of the Muslim calendar is marked by the date that Muhammad and his followers left his hometown of Mecca to escape persecution.
They made their hijrah (journey) to Medina on July 16, 622, so the Muslim calendar begins with that day.
The Muslim calendar is based on lunar cycles and therefore is either 354 or 355 days long every year. A solar year, in contrast, is based on Earth’s movement around the sun.
The solar calendar has 30 or 31 days per month and is 365 and one-quarter days long. A lunar month typically has 29 or 30 days.
What that means is that what were summer months several years ago are now spring months, and will become winter months over the next several years.
In some countries that use a lunar calendar, an extra month is observed every three years to prevent the months from drifting too far into other seasons.
Most cultures associate celebrations and holidays with certain times of year and want to keep it that way.