Is it "extra", as in on top of the agreed yearly salary?

Norway has a similar thing, in that 10%+ of your salary gets paid out in a lump sum before the summer holidays, and the tax deducted in December is half what it is in the other months. But it's all just an adjustment in how salaries are paid out and when the tax is paid, so you end up with exactly the same total amount.

Of course either way, employers factor it in.