It’s considered back luck to celebrate a birthday before the actual day in Germany (and even to give a present beforehand). However, it is allowed to “reinfeiern”. Now this is a really difficult word to explain in English because we don’t normally do it! So what could we say?
Would you agree with me that it means getting together on the evening before the actual birthday, anniversary or whatever and continuing to celebrate until the early hours of the next day?
See here what Deutsche Welle has written about the topic.