Beethoven's birthday is considered an important holiday by Schroeder. He first celebrated it in 1953, then celebrated it almost every year afterwards, on December 16, the birthday of Ludwig van Beethoven.
Schroeder is the only person who really celebrates Beethoven's birthday, since he idolizes the composer. Lucy also celebrates it, but only to impress Schroeder. Normally, she is unable to impress him, and sometimes ends up hating Beethoven's birthday, since Schroeder does not care if Lucy likes Beethoven or not. However, in a strip from 1984, Schroeder kisses Lucy on the cheek because she gives him a picture of Beethoven. Lucy has tried to commercilize the day by selling T-shirts, which makes Schroeder upset, because he wants people to enjoy Beethoven's birthday for Beethoven, and not for merchandise.
A running gag connected to Beethoven's birthday has Schroeder holding up signs to remind everyone that Beethoven's birthday is coming up. For instance, the sign says, "5 more days til Beethoven's birthday". In one strip Lucy tells Schroeder that the exact date of Beethoven's birth is not known. Schroeder then holds up a sign that says, "10 more days til Beethoven's birthday (more or less)".
In a strip from 1957, Schroeder forgets to celebrate Beethoven's birthday. The following week, he is walking around ashamed that he forgot it. He has a hard time looking at the bust on top of his piano.