Michelle Obama turned 50 a day ago but the big celebration is taking place at a White House dance party Saturday night.
The White House has said nothing about the guest list or about how many people are expected or about who's providing the entertainment.
But word has spread that President Barack Obama has told guests to come prepared to dance.
Mrs. Obama said last month she wasn't certain how she would spend her birthday but that whatever she did might involve some dancing.
"There's one thing I can tell you for sure ... people will move," Valerie Jarrett, a senior White House adviser who is close to the Obamas, told The Associated Press.
The buzz is that Beyonce, who's friendly with the Obamas, will sing. Beyonce's husband, rapper Jay Z, performed in Washington on Thursday night.
The first lady spent her birthday largely out of sight, though there was a Twitter post in which she showed off her new AARP card.