Eid al-Adha in 2017 is on Friday, the 1st of September(1/9/2017).
Note that in the Muslim calander, a holiday begins on the sunset of the previous day, so observing Muslims will celebrate Eid al-Adha on the sunset of Thursday, the 31st of August.
Although Eid al-Adha 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 Eid al-Adha moves in the Gregorian calendar approximately 11 days every year. The date of Eid al-Adha 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 Eid al-Adha. 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.