Originally it was a week-long series of TV shows featured on the Discovery Channel. It's been running since the 80s I think, and is super popular at the moment because it has been promoted loads on Facebook and Twitter.
Like many social media trends, it's been turned into parody:
People now also use the term "shark week" to describe the week of each month that a woman might be likely to act a bit like this:
That's right, her period!
It's also not only because of the temper and bloodfest that they call it shark week. It also has some biological relation;
Here's some evidence of its use on Twitter:
Hope this clears things up!