According to unconfirmed (although very likely) information Steam Summer Sale 2017 will start on 22nd of June. Last year Steam Summer Sale has started on Thursday, June 23rd, therefore this year should be no difference to what we had in the past. As always majority of the games will go with worthy discount (except fresh titles or E3 announced games).
Steam Summer Sale 2017 Trading Cards
As always Steam has included a community event with Steam Trading Cards. They are already available to get just like one year ago: one week before the sale starts all crafted badges will grant you one random Steam Summer Sale event card. This is a good time to craft all the badges you have because receiving an extra card is better than a coupon for a shitty game (yes, almost every time the coupon is for trash game).
What to expect?
This Steam Summer Sale is probably going to have nothing other than regular sale offers. There might be added some sort of community votes and titles with biggest vote count may get better discount. Speaking about Flash sales – not gonna happen (these are the sales available for 8-12 hours and then dissapear). Latest steam refund policy ruined this concept as people who bought the game before flash sale will be asking for a refund and Steam support will basically die answering those requests.