Justin Timberlake, Jessica Biel Make First Official Post-Engagement Appearance at MET Gala

Jessica Biel and Justin Timberlake attend the "Schiaparelli And Prada: Impossible Conversations" Costume Institute Gala at the Metropolitan Museum of Art on May 7, 2012 in New York City. Credit: Kevin Mazur/WireImage.com

They rarely walk the red carpet together, but future spouses Justin Timberlake and Jessica Biel always seem to make an exception for the annual  Met Gala in New York City.

For the first time since their winter engagement, Timberlake, 31, and Biel, 30, made their first joint appearance at the Metropolitan Museum of Art in NYC Monday. Biel showed off her flawless figure in a plunging white Prada dress with cascading jade green stones. "Prada picked it out for me," she told reporters. The brand also outfitted Biel with her shoes and clutch.

Biel, of course, also showed off her diamond engagement ring, which cost an estimated $130,000. It was designed by Timberlake, who "went rogue without telling Jessica's stylist" to create the bauble, a source recently told Us Weekly.

Timberlake, meanwhile, suited up in a black Tom Ford tuxedo with a satin shawl lapel, evening shirt, satin bowtie, cufflinks and leather "Chelsea" boots. "We're gonna go find champagne," the singer told reporters on the red carpet.

The loves of five years are in the midst of planning their summer nuptials, sources recently told Us. "It will be a big wedding. They have lots of people they want to invite."

Bride-to-be Biel has even enlisted help from her future mother-in-law, Lynn Harless. "She's helping Jessica with every little thing," a source said.