I admit that I originally predicted that the sale would start today, but I think that I will have to change my prediction to Tuesday (we'll see if I regret it in a couple of hours). Tuesdays seem to be a more popular day to start sales recently, plus Summer Game Fest is still going on and it might be harder to spread the word while it is taking up most gaming media space. If not Tuesday, then it should be Wednesday because I would be surprised if the summer sale did not start this week.
As for summer sale itself, maybe I am just setting myself up for disappointment but I am quite excited for it. If Darkest Dungeon 2, a relatively big deal I think, can be released before summer sale starts then I am quite excited to see what GOG has saved up to release during the actual summer sale.