Summer Picnic Sale is a card set that is related to the Summer 2016 Steam sale. The cards became available on June 23, 2016, the same day as the start of the sale. Prior to that point, it was possible to obtain cards from the Mysterious (2016) set by crafting badges, but these card turned into cards from the Summer Picnic Sale set when the sale started.
Cards from this set can be obtained by crafting badges from any game, making purchases on Steam (approx. $10 US per card) during the sale, and fully exploring the Discovery Queue (up to three times per day - account Level 5 or more required).
The level 15000 badge is identical to the level 8, as is the level 17000 and level 7.
The sale badge originally had a bug where the highest leveled badge one could craft was 32,767, the maximum value contained in a short integer. The bug was eventually fixed, allowing users to craft badges above level 32,767.