"This has been the wildest year of my life... 28 I will never forget you. Here's to making 29 even better with my people!" Peter captioned a photo in which the guys posed on their knees and the ladies stood behind them smiling big.
The photo also included three adorable dogs, one of which Peter was holding in his arm.
"Amazing!" Kelley commented on the post. "Love celebrating you."
And Peter replied to Kelley's post with a red heart.
Peter also shared an Instagram video in which he read one of his birthday cards out loud and hung out with Kelley poolside.
Next to the pool, Peter's family apparently decorated a big wall of light blue and white balloons with a sign that read, "Happy Birthday."
"Best present ever, right here," Peter said of Kelley, who apparently printed out special photos of the couple by his wall of balloons.
But Kelley also apparently got some alone time with Peter on Monday, one day before the pilot's actual birthday. She documented a few cute moments on Instagram.
Kelley added that she and Peter motivate each other to be better and they've become best friends. She also apparently doesn't hold Peter's past and dating history against him.
"We've just done a good job of reassuring each other and just explaining why I did things, and owning up to my faults and just being a better person moving forward," Peter noted.
He later added in the ET interview, "Everything I put Kelley through... I'm still working to make it up to her for not picking her, and I'm never going to stop... I truly believe she is the perfect person for me."
During The Bachelor's live Season 24 finale on March 10, Kelley, who was sitting in the audience, watched an indecisive Peter break off an engagement to Hannah Ann and then reconcile with his supposed true love, Madison, whom he officially split from on March 12, just two short days after the finale aired.
However, Peter insisted during his ET interview that he and Madison never officially got back together after the finale and they just spent a couple of days talking.
Peter and Kelley were first spotted together hanging out in Chicago in late March.