In many countries, birthdays are an occasion to celebrate that the person involved has survived another year. Germany is no different in this respect, but the traditions there are a bit different than what people in the UK or in the United States might be familiar with!
First: never wish someone happy birthday before the actual date!
From what we can tell, this ties into a superstition that the person might die before they reach their special day. On the bright side, this is where the German love of punctuality shines; calendars are meticulously kept of who has birthdays and when. Or you could rely on automated calendars like your phone or Facebook to do it for you, now. Just never, ever wish people a happy birthday before the day. Don’t say it, don’t give them presents, nothing that might be construed of as wishing them a happy birthday before the actual date. By extension, this means that a birthday-party cannot happen before the day itself, either. (Speaking of happy birthday: you say “Alles Gut zum Geburtstag!” when you DO want to wish someone a happy birthday, in German.)
Second: if you get invited out to someone’s birthday-party, you are THEIR guest.
This means two things. One: adults in Germany organize their own (if any) birthday-party shenanigans. Two: if you’re the one being invited out, you don’t pay for anything. The host is supposed to treat their guests. This is against some expectations in the US – in the US, getting together for a birthday-party or dinner is often organized by friends of the person having their birthday. However, this only seems to apply to adults: if you’re a child in Germany, expect to be treated very well on your special day up till about age 12.
Third: speaking of children…
Some families will put candles on a “birthday-wreath” made out of wood (Geburtstagkranz) – these wreaths have about ten or twelve holes in them, meant to represent each year as a child. However, the custom of putting candles on a cake happens in Germany, too! Just don’t be surprised if you see these wooden wreaths used instead.
Fourth: Old traditions die hard!
There are some birthday-traditions related to “helping” the person find a match; for example, in northern German areas, there is a custom of having unmarried men sweep a public place or hall on their thirtieth birthday. Unmarried women have to clean doorknobs (often with a toothbrush). This has roots in announcing that there is an unmarried person in the community that can clean, as sort of a desperate attempt to find them a match. Strengthening this tie is that the only way to be “freed” from these chores is to get a kiss from the opposite sex. (The 25th birthday custom of the “sock wreaths” and “carton wreaths” for men and women, respectively, also seem to say to the entire town: here’s an old man at 25, or a spinster at 25.) What will happen to these traditions now that people are generally marrying later and later, who knows.