Update: Mysterious cards have now appeared.
Just like last year when the Steam winter sale date was leaked, an image shared with Reddit appears to spoil the summer sale window. In the screenshot (part of which is shown above,) an anonymous game developer asks about Valve’s spreadsheet for setting up discounts on their games.
The specific query isn’t all that relevant for a wider audience, but the mention of 13 June as the deadline for submitting the game discounts is an indication of when the Steam summer sale may well take place.
Last year, the winter sale deadline was 15 November and the sale started on the 27th. Applying a similar model, that would put the summer sale in mid June.
The last couple of summer sales have started in mid July, but with parts of Valve busy organising and hosting Dota 2’s International 4 tournament throughout much of that month they may wish to avoid a clash and get the sale underway in June.