The date of Arafat Day varies because it is based on the Islamic Hijri Calendar. The day is on day number 9 in the islamic month dhu al-hijjah.
The Islamic Hijri Calendar is a lunar calendar, which means that the calendar depends on the lunar phases. This means that, in the next ten years, Arafat Day varies between March 10 and June 15. In the table below you can see the date for Arafat Day in the coming years in the western calendar (Gregorian calendar).
