Santa Claus is a jolly man who lives at the North Pole with elves and reindeer. His job is to bring gifts to the homes of good children during the late evening and overnight hours of Christmas Eve or on December 6th, his Feast Day in the Calendar of saints.
During a "summer run", Santa made use of the Chimney Vator, one of the many systems the O.W.C.A. has in place to transport its agents to and from their lairs around the world. During the ride, he met Agent P. Santa saved Agent P later in the day by catching the electromagnet on his sleigh that Dr. Doofenshmirtz had released from a crane, intending to crush Agent P. For this "summer run", Santa did not require the full complement of reindeer to pull the sleigh ("I, Brobot").
On Christmas Eve, Phineas and friends finish delivering gifts across Danville and find the sauna being used in the rest stop for Santa that Phineas and Ferb built atop the Flynn-Fletcher house. Santa chuckled, seemingly expecting for all of this to happen. He also reads Phineas' letter, saying all Phineas wished for was to be like him. Santa is also Phineas' greatest hero, delivering gifts to everyone in one night, something even he possibly couldn't do. He then takes Phineas' gift with him, thanks the kids for doing Danville for him then leaves. Santa is also revealed to be the one responsible for creating the plans for the Naughty-inator which Doofenshmirtz is using to get Danville into the naughty list, as part of his plan to grant Heinz's wish of gaining the ability to hate Christmas, which has succeeded, much to Heinz's delight, declaring this as his best Christmas ever. The next morning, Phineas and friends find gifts under their trees, all from Santa ("Phineas and Ferb Christmas Vacation!").