Beyond Selidor

I'm not sure it's unique to Germany, but I do know that it's not a thing in either Australia, where I grew up, or the UK, where I live now. For example, my birthday is 20th December, which in Australia always falls smack in the first week of summer holidays — so my birthday parties were always on the Saturday before my birthday, when I was a child, and no one thought this was odd. People in both places tend to celebrate birthdays on the closest convenient weekend, whether that's before the birthday, on the actual day, or afterwards.