The pop up is in game. Double experience In /cal it says "Sept 2 at 1 am to Sept 7 12 am you'll earn double experience Some Lon Packs on sale in marketplace. So it appears we and the game have beat the powers that be that were supposed to tell whoever to post this.
CatsPaws, your times are only correct for your time zone. For Pacific Daylight Time, which is what Darkpaw works on it is: Friday, September 2nd 00:00 PDT to Tuesday, September 6th 23:00 PDT