Earlier today, two Steam database received updates hinting at an upcoming sale.
The first change was to the database dealing with the titles shown in the store. A massive swathe of summer themed bundles have been added to this database, with “June 2013” featured in many of the titles. This indicates that whatever these bundles are, they’re linked to some sort of event that is expected to start soon. The database update also added several packs that come with multiple copies of a title. In the past, the sale often features such bundles, so this is another indication that something is on the way.
The second update was to the Steam Marketplace, which is used to buy and sell game items and Steam Trading Cards. A “Mystery” trading card is apparently the only addition from the update, with users seeming to find it after crafting their cards together. Given that last week Valve stated that the release of the Trading Card system would allow something exciting to happen, it’s quite likely that this mysterious card is also related to the Summer Sale.