Summer Sale trading cards are only available during the Summer Salien Event. There are six ways to get them:

Play the Salien Game
You can earn up to three trading cards each day by playing the Summer Salien game. Have fun and earn some rewards while you're at it!

Craft a Game Badge
Crafting a badge for any game during the sale will get you an additional Summer Sale trading card.

Make a Purchase
As an extra reward for purchasing, for every $10 USD spent during the sale, you’ll receive a Summer Sale trading card. From your Summer Sale badge page select

Go through your Discovery Queue
If you’re Steam Level 5 and above, you’ll get a Steam Summer Sale trading card once you view all the items in your daily Steam Discovery Queue. You can continue and go through your Discovery Queue two more times each day to get a maximum of three card drops each day. (If you reach Steam Level 5 before the end of the sale, you’ll be granted the cards you earned by exploring your Steam Discovery Queue during the sale).

Trade for them
That’s why they’re called trading cards! Find friends with other cards and make trades. Your summer sale badge page shows which friends have the cards you need.

Buy them from the Community Market
Sell the cards you don’t need and buy the ones you do, all with other Community members. Turn those duplicate cards into Steam Wallet funds to buy games.

Remember: Along with the sale is the Summer Saliens Game. Team up with other Saliens to fight The Duldrumz on different planets and free the abducted games. Gain XP as you battle, level up, unlock new abilities, and win cosmetic items to deck out your Salien. Plus, get Summer Sale Trading Cards just for playing.

The Steam Summer Sale 2018 has begun

The prophecy foretold is now fulfilled: The Steam Summer Sale is live! That means cards to collect, a badge to earn, a game to play, and yes, some deals on games.

This year's event includes a game, Saliens, that pits players against the Duldrumz, "purveyors of boredom of the highest order," who are out to steal all of the games and hide them on distant alien worlds, where you won't be able to find them. But have no fear, the Saliens are here! Under your control, they'll do battle on various alien worlds, defeating enemies, taking control of map sectors, earning experience, and claiming rewards (including free games) for victory.

As usual, you can earn three free Steam trading cards each day by browsing your Discovery Queue, which can be spun into a badge once you've got them all.

The 2018 Steam Summer Sale will run until 10 am PT/1 pm ET on July 4.

The word on the street, courtesy of the Steam Database Twitter account, is that the legendary Steam Summer Sale will start today.

"Mysterious" Steam cards appeared on the site last week, bolstering the likelihood that the leaked date is accurate, but of course it is: Steam sale date leaks are as regular and predictable as the sales themselves, and I can't remember the last time a leaked date was wrong. Precedent also backs it up. Last year's Summer Sale started on June 22, and in 2016 it was June 23. You might see a pattern emerging there.

Steam Summer Sales are usually big ones, so you can probably expect the queue-browsing and card-collecting activities we've come to expect alongside the usual bevy of bargoons.

The Summer Sale Cards are now available.

You can get the Mysterious Cards before the sale if you buy them on the Steam Community Market or craft badges - instead of a Steam Coupon you will get now a Mysterious Card!

Note that you can't craft the badges before the sale startd at Jun 21st.