Contrary to belief, they do celebrate one another's birthday.
Gilbert's day is one where he wakes up to an extravagant breakfast made by Roderich before spending the day watching movies on the couch, only going to a club to celebrate with his brother, Roderich and others.
Roderich's is more relaxed. He wakes up to a small breakfast in bed, a little bit of snuggling, before going out to visit the Mozart's House in Vienna. Later on, of course is a symphony performance with Gilbert right at his side.
