From /r/steam
Q: What are these Summer Sale cards and how can I obtain them?
A: Sale cards are trading cards that you can only get during a Steam Sale, in this case: the Steam Summer Sale. These cards expire and can no longer be used once the Summer Sale is over.
You get one for each badge you craft during the sale. They are untradable/unmarketable for 7 days if the trading cards used to make the badge that gave you a sale card are untradable/unmarketable.
You can also obtain them from buying $10 USD worth of games and/or from completing Discovery Queues. Cards from these examples are instantly tradable.
Keep in mind that your account must be at least level 5 in order to get card drops from completing Discovery Queues and that at the store resets daily at 10:00am Pacific Time (3AM AEST).
Enjoy the free money! :)
You get 3 per day.