Looks like the official F/GO USA Twitter just announced the 3rd anniversary stream for July 6th, 6 p.m. PST. Voice actor Kenji Akabane (who voices Kadoc Zemlupus) will be joining in.
You can expect the 3rd anniversary celebration (and hence GSSR and Skadi banners) to take place thereafter. Not too surprisingly, this is a separate stream entirely from the previously announced Aniplex stream on the 4th-5th, since the Fate Grand Order game was not listed anywhere in the schedule for that stream, only the animes.
Double edit: Aniplex stream stated 8 p.m. PDT on their promo card for FGO. So combined with the announced maintenance times (from 5-9 p.m. on July 6th), it’s most likely going to be 8 p.m. after all.