Credit: Robin Platzer/ Twin Images

Monday certainly wasn't just any old day for Giuliana Rancic.

That morning, the E! News hostess and reality star, 37, announced her decision to undergo a double mastectomy later this week. Rancic revealed a breast cancer diagnosis back in October, and the disease was initially treated with a lumpectomy and radiation.

And despite her very emotional appearance (with husband Bill Rancic alongside her) on The Today Show explaining the big news, Rancic didn't call in sick for work.

Later that night, she was back on the job -- meeting Angelina Jolie, Brad Pitt and other stars on the red carpet for the NYC premiere of In the Land of Blood and Honey, Jolie's intense debut as a director and screenwriter.

According to an onlooker, the radiant Rancic looked "happy and upbeat" chatting with the stars and fans.

"Overall, she's in unbelievably great spirits," a source close to Rancic told Us Weekly. "She's been working a lot. It keeps her motivated."

Giuliana and husband Bill, 40, thanked fans via Twitter late Monday. "Thank u all! Thank u for the outpouring of prayers."