Farm opens at 10am Pacific, which should be 11am mountain 12 pm central and 1pm eastern also 6pm for the ladies and gents across the way...if I've got my time zones right. Keep in mind that vendors may or may not be active as this is a glorified stress test to see if it can handle up to 26 players at once.
EDITED: correction to mountain and central times as well as adding UK time.
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And Bungie has said that no vendors will be active