According to Delish, it's set to arrive nationally on August 31, 2016. The drink was first introduced nationally back in 2013 but has only returned seasonally to certain areas since.
McDonald's Pumpkin Spice Latte is made with a combination of whole or nonfat milk, pumpkin spice-flavored syrup, and espresso (Here's my review of it from back in 2013).
Photo via McDonald's.