Steam Summer Sale 2017 is a special card set that was made available during the 2017 Summer Sale. The cards initially became available on June 15, 2017, though were not craftable into a badge until the sale began on June 22, 2017.
There are 10 cards in the series. You will also earn a card each time you use your Discovery Queue, up to 3 times per day. You can also earn cards by crafting a badge from any other card set - doing so will give you one card at random in addition to the normal badge crafting rewards. Additionally each $10 spent from buying games will grant you one card.