The Jalali calendar, also known as the Jalali solar calendar, is a solar calendar that was introduced in the 11th century by the Persian mathematician and astronomer Omar Khayyam. It is based on the Earth's revolutions around the Sun and consists of 365 days in a common year and 366 days in a leap year. The calendar is known for its accuracy and is used primarily in Iran and Afghanistan. The Jalali calendar begins on the vernal equinox, marking the start of spring in the Northern Hemisphere. It divides the year into 12 months, with the first six months having 31 days, the next five months having 30 days, and the last month having 29 or 30 days, depending on whether it is a leap year. This structure allows the calendar to stay in sync with the seasons.