The cover below is an example of this - a Christmas themed cover with a special postmark from Santa Claus, Indiana.
Yes there is such a place as Santa Claus in the US. Apparently the town of Santa Fe, established in 1854, was denied its application for a post office since there was already a Santa Fe recorded with USPS. A town meeting was held and the townspeople elected instead to rename it as Santa Claus.
It is the only post office in the world to bear the name and as such receives thousands of letters to Santa from all over the world. Every year a Christmas pictorial postmark is chosen from designs submitted by art students from Heritage Hills High School.
On the cover are two stamp issues from October 2012. The first is a Forever stamp (45c) depicting the flight of the Holy Family to Egypt. Interestingly enough, this stamp was unveiled at the Bethlehem Chapel of the Washington National Cathedral.
The second stamp is actually a composite of four self-adhesive Forever stamps featuring Santa and his sleigh circling around snow-covered rooftops.