I'm skeptical like SCPM. That'd be 6AM Poland time, and I think GOG would want staff up, awake, and focused for such a big sale with big new releases happening. I could be wrong though of course.
I agree with tfishell. It would be quite risky to launch a big event at Sunday midnight. Especially with GOG's tradition to make some mistakes, price bugs, missing announcements or incorrect sales' time-frames.
So I would not
expect Summer Sale in 2 hours, but perhaps we can get some hints if e.g. there will be no new Weekly Sale? I'd rather bet for Monday 15:00 p.m. Polish time (when current flash deals expire) or even Wednesday (Warhammer sale ends in 2d 20h and its date is the last one from all active at the moment).