NianticGeorge on TSR says probably sometime during business hours Pacific Daylight Time. No word on how long they will be down. My very uninformed guess is that there will be an update released Monday that necessitates gym shutdown until all players get access to the update. These things usually hit the Apple store on Thursday, so that’s my guess as to how long the gyms will be down. I’ve heard other guesses out there, too (one that they won’t be back until the Chicago anniversary event, but I think that would be unlikely).