The melody of "Happy Birthday to You" comes from the song "Good Morning to All", which was written and composed by American siblings Patty Hill and Mildred J. Hill in 1893. Patty was a kindergarten principal in Louisville, Kentucky, developing various teaching methods at what is now the Little Loomhouse; Mildred was a pianist and composer.
The sisters created "Good Morning to All" as a song that would be easy to be sung by young children.
It is often the tradition that at a birthday party, the song "Happy Birthday to You" is sung with the birthday person seated in front of a table where there is a birthday cake with candles that have just been lit, with the other guests gathered around. The number of candles is often the same as the age of the birthday person. After the song is sung (usually just once), sometimes party guests will add phrase like "And many happy returns!" or "And many more!" expressing the hope that the birthday person will enjoy a long life. The birthday person is asked to make a wish ("Make a wish!") -- which is done silently—and then blow out the candles. Traditionally, the blowing out of the candles is felt to signify that the wish will come true. Once the candles have been blown out, people often will applaud, and then the cake is usually served—often by the birthday guests—and eaten. Often, after the cake is eaten, each guest gives a gift, usually wrapped in festive paper, to the birthday person. Often the birthday person will then open the gifts, revealing their contents to all. That usually concludes the ritual aspect of a birthday party, which then proceeds much like any other but with the birthday person being treated as the guest of honor. (source: Wikipedia)