1. No one mentions your birthday until it’s your birthday.
In Germany, mentioning someone’s birthday before the date is considered bad luck. If your German friends are nice enough to give you a birthday present, don’t open it until your actual day. Or at least don’t tell them you opened it before your actual day.
If you’re sending someone a gift, make sure to write “GEBURTSTAG” in all-caps, red-alert style, so they don’t inadvertently open it a few days early and bring unknown horrors upon themselves.