The lunar year used to determine Ramadan, hajj, and other Islamic dates is the Hijri calendar.
It is also called the Islamic or Muslim calendar. Its months begin with the lunar cycle, and a common calendar year has twelve months totaling about 354 or 355 days. As a result, Islamic months move through the seasons of the Gregorian solar year.
The calendar era begins with the Hijra, Muhammad’s migration from Mecca to Medina in 622 CE. The first year of the era is conventionally designated 1 AH, meaning Anno Hegirae, or “in the year of the Hijra.” The calendar’s months include Ramadan, Shawwal, and Dhu al-Hijjah.
Actual religious observance can begin on different Gregorian dates in different places because communities may use local moon sighting, astronomical calculations, or an established calendar. This explains why a single Islamic date can vary by location without changing the lunar system itself.