Recently, I Exposed more and more to the 11.11 singles day commercials.
I got the impression it's a massive black Friday shopping day.
Can someone please explain me what is the deal behind that day?

It's a relatively new custom, popular in China, for unmarried people to celebrate their "singleness." See: https://en.wikipedia.org/wiki/Singles%27_Day

Yes, it is a very popular Asian holiday (they have done it for years) You may compare it more to the American Valentine day. (not really celebrated in China))