When is Waqf al Arafa in 2015? 

Waqf al Arafa in 2015 is on Tuesday, the 22nd of September.
Note that in the Muslim calander, a holiday begins on the sunset of the previous day, so observing Muslims will celebrate Waqf al Arafa on the sunset of Monday, the 21st of September.
Although Waqf al Arafa is always on the same day of the Islamic calendar, the date on the Gregorian calendar varies from year to year, since the Gregorian calendar is a solar calendar and the Islamic calendar is a lunar calendar. This difference means Waqf al Arafa moves in the Gregorian calendar approximately 11 days every year. The date of Waqf al Arafa may also vary from country to country depending on whether the moon has been sighted or not.
The dates provided here are based on the dates adopted by the Fiqh Council of North America for the celebration of Waqf al Arafa. Note that these dates are based on astronomical calculations to affirm each date, and not on the actual sighting of the moon with the naked eyes. This approach is accepted by many, but is still being hotly debated.