Or will it end during the middle of the day?
It’s now after the reset and it’s still active, so I would imagine it will last until tomorrow’s reset. But I can’t answer with 100% certainty.
I don’t think Blizzard ever gave an official time, but the two most likely options are:
(1) It ends at midnight PDT.
(2) It ends at reset tomorrow morning (8 AM PDT)