If you complete 5 of 6 tasks in one week, all are active, then you complete the 6th the following week, so all 6 are then active, does this open up the Legendary task?
No. The timers on completed tasks do not affect Legendary task availability; otherwise legendary tasks would just stay active until the timers on the 6 regular tasks ran out. 
Thank-you. So if I am reading your answer correctly, you have to complete them all in the same week.
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Yes, that is correct. The 6 tasks reset at the end of the week regardless of if you’ve completed them or not (any bonuses you’ve gained during said week continue into the next week however for however long you have left on them).
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Trust me you aren’t missing much 
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Good to know 
Lengendary are mediocre at best.
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Agreed. I wouldn’t bother unless your guild has a huge surplus
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