Xur shows up at the daily reset on Friday and leaves at the daily reset on Sunday. This is the standard from Xur, as well as most events and activities within the game. The only activity that normally didn't follow the appears/disappears at reset was Trials of Osiris and Iron Banner, which were both competitive PvP and started later in the day past reset. That is the normal time, as far as the source; past experience. As far as if Xur will appear at reset time this time, all the sources online (sites that report on him) say he'll appear at reset like normal, but none of them provide any sources for that information, so they may either have direct Bungie sources, Bungie announced this and I can't find it quickly, or they're going based on past experience.