OMG, it’s here. I think they finally announced it because the Chicago event leaked on TSR. Solstice Event with Fire and Ice pokemon starts June 13 (with XP bonus for accurate throws … and egg hatches reported earlier, but not in the above link). “Soon thereafter” is a major update involving “collaborative group gameplay.” Gyms will be disabled for a short period of time leading up to this. NianticIndigo on TSR says all gym defenders will be returned to their trainers during that time period. A Chinese translation of the announcement says this will happen June 16.
Then July 22 a major Pokemon event at Grant Park in Chicago.