I saw on a tech forum that a former Bungie employee said that the reason we didn't have an exotic Weapon in Xur's inventory this past week is because he was supposed to have Gjallarhorn in his rotation but they decided to wait until TTK to release it again. So instead they removed it and replaced it with celestial nighthawk. I would have much rather had Gjallarhorn but that just my opinion.
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Proof!! Xur just got Nerfd in advance. Bungie forgot to mention it. ;)