Xür, a warlock, is a weekly event that comes to the tower every Friday to Sunday morning. He gives out exotic engrams and weapons and armor pieces. He trades these items for consumables called strange coins. Strange coins can be awarded to you after a crucible match, or a package from one of the many vendors in the tower. They are mostly awarded by doing the Weekly Nightfall. These are extremely hard heroic strikes chosen from different planets such as Mars, and Venus and etc. Xür can have a chance to sell a weapon to you every other weekend, according to some myths and theories from many players.