Steam is well known for doing amazing sales in Summer and Winter. But they do try to keep the when very hush hush. However, yesterday the dates for the sale were reportedly leaked to the public.
While these dates can change as of now what we have on record shows the steam sale will occur June 23, 2016 through July 4, 2016.
Steam themselves have not confirmed or denied the dates as of now. I would expect it to be somewhere in that time frame for the start date.
As of now there are also no official titles affected by the summer sale that is coming. We will update the post with more information as it comes.