It is one of the most beloved and famous of all songs, belted out at countless gatherings for infant and octogenarian alike. Yet “Happy Birthday to You,” far from being as free as a piece of cake at a party, is actually considered private property.

A federal lawsuit filed by a group of independent artists is trying to change that, and lawyers in the case, in a filing last week, said they had found evidence in the yellowed pages of a nearly century-old songbook that proves the song’s copyright — first issued in 1935 — is no longer valid.

A judge may rule in the case in coming weeks. If the song becomes part of the public domain, it would cost the Warner Music Group, which holds the rights, millions of dollars in lost licensing fees. It would also be a victory for those who see “Happy Birthday to You” as emblematic of the problems with copyright — a song that has long since survived anyone involved in its creation, yet is still owned by a corporation that charges for its use.