The Charmed star's big day is on Wednesday (19Dec12), and Milano's agent husband David Bugliari organised the secret bash at The London hotel in Los Angeles to ensure she celebrated the milestone in style.
The event was themed like a school prom because the star missed out on her own right of passage party in the 1990s.
A source tells Us Weekly, "It's prom-themed because Alyssa didn't have a prom and hers would have been in (the) 1990s. Everybody dressed up and everyone got a corsage when they walked in."
Guests reportedly included rapper Ludacris, movie producer Brian Grazer and funnyman Ken Jeong.