[If you've ever wondered where this came from -- which I'm sure you have...]
"Happy Birthday to You" is a song which is sung to celebrate the anniversary of a person's birth. According to the Guinness Book of World Records, "Happy Birthday to You" is the most popular song in the English language, followed by "For He's a Jolly Good Fellow" and "Auld Lang Syne". The song has been translated into many languages, though it is often sung with the English lyrics in countries where English is not a primary language. It's also the most frequently sung song in America.
The melody of "Happy Birthday to You" was written by American sisters Patty and Mildred Hill in 1893 when they were school teachers in Louisville, Kentucky. The verse was originally intended as a classroom greeting entitled "Good Morning to All". The version as we know it was copyrighted in 1935 by the Summy Company as an arrangement by Preston Ware Orem, and is scheduled to expire in 2030. This was the first copyrighted version to include the lyrics. The company holding the copyright was purchased by Warner Chappell in 1990 for $15 million dollars, with the value of "Happy Birthday" estimated at $5 million.  While the current copyright status of the song is unclear, Warner claims that unauthorized public performances of the song are technically illegal unless royalties are paid to them. It is not completely certain who wrote the lyrics to "Happy Birthday to You".
During the SARS outbreak in Hong Kong in 2003, the government advised people to regularly wash their hands with soap for around 15 seconds. In order to make this practice more easily handled by kids, some primary school and kindergarten teachers told their students to sing "Happy Birthday to You" slowly while they washed their hands, and to only stop washing after finishing the song.
"Happy Birthday to You" copyright status
There is a 1935 copyright registration for "Happy Birthday to You", as a work for hire by Preston Ware Orem for the Summy Company (the publisher of "Good Morning to All"). "Good Morning to All", however, was published in 1893 and is public domain by U.S. statute. The current owner of the 1935 copyright believes that one cannot sing "Happy Birthday to You" lyrics for profit without paying royalties. Except for the splitting of the first note in the melody "Good Morning to All" to accommodate the two syllables in the word happy, melodically "Happy Birthday to You" and "Good Morning to All" are identical.
Neither the words nor the music of "Good Morning to All" are copyrighted under U.S. federal statute.
In 1924, Robert Coleman included "Good Morning to All" in a songbook with the birthday lyrics as a second verse. Coleman also published "Happy Birthday" in The American Hymnal in 1933. Children's Praise and Worship, edited by Andrew Byers, Bessie L. Byrum and Anna E. Koglin, published the song in 1928.
Later the "Happy Birthday to You" lyrics combined with the Hills' published melody showed up on stage. The Broadway musical The Band Wagon used "Happy Birthday to You" in 1931. There was no copyright for the Happy Birthday lyrics at the time. Contrary to what is often erroneously reported, the lawsuit was dropped, and there was no outcome to the case. As a result, the Summy Company registered the copyright for Happy Birthday to You, which does not affect today's public domain status of "Good Morning to All."
Precedent (regarding works derived from public domain material, and cases comparing two similar musical works) seems to suggest that the melody used in "Happy Birthday to You" would not merit additional legal protection for one split note.
Whether or not changing the words "good morning" to "happy birthday" should be protected by copyright is a different matter. The words "good morning" were substituted with "happy birthday" by others than the authors of "Good Morning to All".
An interesting earlier songbook is The Golden Book of Favorite Songs (Chicago: Hall & McCreary, 1915). It includes the song "Good Morning to All" printed with the alternate title: "Happy Birthday to You." However, the "Happy Birthday to You" lyrics are not actually printed along the staff.
Regardless of the fact that "Happy Birthday to You" infringed upon Good Morning to All, there is one theory that because the "Happy Birthday to You" variation was not authored by the Hills, and it was published without notice of copyright under the 1909 U. S. copyright act, that the 1935 registration is invalid.
Outside of the United States both the melody and the words are protected by copyright in those jurisdictions with a copyright term of length of life of the author plus 70 years. Of the two co-writers of the melody, Patty Hill's life determines the length of copyright as she died decades after her sister in 1946. The lyrics on the other hand are protected with reference to their writer Preston Ware Orem who died in 1938. In life of the author plus 70 years jurisdictions the lyrics will come out of copyright at the end of 2008 and the music will come out of copyright at the end of 2016. In those jurisdictions which remain life of the author plus 50 years for determining copyright both lyrics and music are already out of copyright.