Shannon, 54, enjoyed a "#boozybrunch" with several of her gal pals that she referred to as her "#oldestfriends".
The mother-of-three doesn't officially turn 55 until Monday, March 25.
Her RHOC costar Kelly Dodd possibly experienced some FOMO from the birthday outing. "Now that's a fun crew!" she wrote in the comments section.
Shannon has been doing a lot of celebrating lately. She shared a rare photo of her father alongside her sister for his 90th birthday on Tuesday, March 19.
She also reached her goal weight of 40 pounds last month.
Shannon's dramatic weight loss was visible when she recently sported her iconic St. Patrick's Day shirt that barely fit her anymore.