Hold on.
So, Skullgirls is free to play over the weekend with a 33% discount.
The Squiggly DLC is free until the 22nd.
In Steam, DLC is treated the same as any other mod - it's saved as a part of the game's data, and is permanently associated with the account with which it was purchased/downloaded, even if you don't own the game in question. i know this to be true - I won a piece of Awesomenauts cosmetic DLC from Modbot thinking it was for Scribblenauts - an easy mistake to make at 3 in the morning with a cold-induced headache - but then, I downloaded Awesomenauts during one of its many free weekends, redeemed the code, and it worked fine until the free weekend was over. Then about a month later I won Awesomenauts itself off Modbot, installed it, and the DLC was still attached as if nothing untoward happened.
Does that mean that you could download the game, then download the DLC, then not buy the game until, say, a 50% off Daily Deal next month, but the DLC would still be attached to the game data associated with your account, just as if you owned it legitimately? Could be a nice loophole to get Squiggly for free if you don't have the cash to pony up yet (like me).
