Just in case you don't know, he -always- has a Xenology quest.
The only times you won't be able to get one is if you already have a cipher, or if you didn't complete xenology before he disappeared on weekly reset as it will count as a completion on the week afterwards and not the week you got it.
English
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Oh really? OK, This is my third Xenology in the past 2 months. So, Xur had Xenology offers throughout the whole year on weekly basis?
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Yup, there's normally a second page on his shop menu to pick it up.
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I know about the second page but I had no idea that last year I could have had Xenology quests every week . I missed out.