Lea Michele and Cory Monteith celebrate his birthday last night at Soho House. Credit: Jason LaVeris/FilmMagic

Cory Monteith has many reasons to be celebrating! After completing a month-long stay in rehab for substance abuse, Monteith went out with his Glee costar and real-life girlfriend Lea Michele in honor of his 31st birthday on Saturday, May 11.

The couple of more than a year was spotted celebrating Monteith's special day with friends at a private club in West Hollywood. An eyewitness tells Us Weekly that Michele, 26, got dressed up in a strapless gray bustier top and black pencil skirt, and Monteith wore a black suit for the outing.

"A very happy birthday to the best guy I know Mr. Cory Monteith," Michele tweeted Saturday. "Thanks for making me feel like the luckiest girl in the world every day."

"Thanks for all the birthday wishes! It was a great day," Monteith wrote early Sunday morning. "Officially old now."

Michele stayed by her boyfriend's side after he checked himself into rehab on Apr. 1. "I'm grateful and proud he made this decision," she said at the time. After he checked out on Apr. 26, Monteith thanked his fans on Twitter for their "continued support."