The only way to get the Mystery Cards is by crafting badges, purchasing them from the community market, or by trading for them.
When the sale starts in 3 days they will turn into regular cards with which you can carft the new badge. And these cards will also be available through the sale event (usually 3 per day).
atm you can get them only from crafting a badge (you get 1 card) or trading with other people or buying them from the market. But when the christmas sale starts, you're going to get 1 card for every 5USD spent od games and 3 cards per day for free from an event (it's usually just clicking through discovery queue three times).
Trading Card community market
Thanks for the helpful replies, guys 
never heard of it only trading community on steam
I get trading cards for no reason at all? how this event works and when it will be up?
Sale starts in about 5 hours :)
in 3 steps
Choose the game that interests you and play itfor free.
Every game has some quests for which you will receive Soul Gems.
You can exchange your Soul Gems for Steam Wallet top-ups, game keys, CS:GO Skins and other rewards.
You know that on Gamehag you can get real rewards for free, right? Join us, complete walkthrough and get your chest!
I guess you guys heard of them too. With the intention of getting some XP, i will just ask you guys if you know how to get them instead of going for the usual googleing it approach that i'm used to ... you get some XP for answering too, win-win xD Sooo, how does one get those Mystery Cards? Just by crafting badges? Maybe there are some less known methods to get them ;)