Here's the story:
"Many people have asked about the extra "a" in Kwanzaa. The extra "a" was legend to have been added during a pageant celebration during one of the first Kwanzaa celebrations. It is said that seven children were on stage. Six of the seven children each held a letter from the word Kwanzaa. When it was noticed that the seventh child did not have a letter, the extra "a" was added. Today this extra "a" helps to distinctly identify this African American Holiday."

...and here's a link.

denise:^)