Xur is a rather unique vendor in Destiny 2. He will appear at 12 GMT on Friday and stick around until reset on Tuesday every week.
He accepts Legendary Shards in exchange for Exotic Weapons and Gear. He will carry with him 1 Exotic Weapon and 1 piece of gear for each class each visit. He will be in a random spot on a random planet each visit and is actually shown on the map for you to locate him easier.