The first lady is spending the day out of sight, with no public appearances after back-to-back White House events this week.
But she tweeted a picture of herself holding up her AARP card. She tweeted, quote, "Excited to join Barack" in the 50-plus club today . "check out my @AARP card!"
President Barack Obama turned 50 in 2011.
A birthday bash is on tap for Saturday night at the White House. The president has been involved in the planning.
Officials are keeping a lid on party details, but guests have been told 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.
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