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Inside Jennifer Aniston and Justin Theroux's secret wedding

Jennifer Aniston and Justin Theroux are really good at keeping secrets.

The pair tied the knot on Wednesday night at their home in Bel Air, California, in a secret ceremony disguised as a birthday party for Theroux, who turns 44 on Aug. 10. A source now tells ET that wedding plans have actually been "in the works for at least a month."

We're told the couple's A-list guests -- which included her "Friends" co-stars Courteney Cox and Lisa Kudrow, as well as Tobey Maguire, Ellen DeGeneres, Portia de Rossi, Jimmy Kimmel, Howard Stern, John Krasinski, Emily Blunt, Jason Bateman, Will Arnett, Lake Bell, Billy Crudup, and photographer Terry Richardson -- did know the Wednesday night party was actually a wedding, and not Theroux's 44th birthday party as the timing would lead outsiders to believe. However, staff -- caterers, valet, etc. -- were told it was a birthday party. Preparations started in the afternoon, with the wedding cake delivered at 2 p.m., and the pastor arriving at around 5 p.m.

More details are now emerging about the 46-year-old actress' big night, including the pair's incredible wedding singer -- Sia, who sang her hit, "Chandelier." In late May, Sia was photographed out to dinner in Los Angeles with Aniston, Cox, and comedian Whitney Cummings (who was also at the wedding), so it's safe to assume wedding plans were perhaps already being discussed more than two months ago.

The intimate occasion was an obvious success. A source tells ET that guests danced to tunes by Michael Jackson, P. Diddy, Rihanna and Jay Z, and also celebrated Theroux's own personal milestone, singing happy birthday to the "Leftovers" star at around 10 p.m. Aniston's BFF, Cox, was the last to leave the festivities at around 3 a.m., the source adds, right after DeGeneres and Kimmel made their own exits.

Last November, Aniston insisted during her appearance on "The Ellen DeGeneres Show" that she would let her friends know if a party was for her wedding, rather than springing it on them. Aniston and Theroux have been engaged since August 2012, leading plenty of her celeb pals to wonder when her big day will be.

"It's funny actually, our friends Sacha [Baron Cohen] and Isla [Fisher] have shown up for parties, for little gatherings, about a year ago, they would show up in beautiful wedding wear," the "Cake" star recalled. "Even like, the Fourth of July last year, they showed up and Sacha was in a powder blue suit and Isla was in a little cute dress. And we were like, 'Guys, it's the Fourth of July, we're in cutoffs and eating hot dogs. We will actually let you know!'"

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