Weird, I generally assumed most people visualize the calendar as a circle when recalling dates. I've always been good at associating dates with events, but never would have thought the circle had anything to do with it.
I don't think i picture it at all in the same way that synasthetes literally "see" other senses, but I do picture a year as a 2d row divided into rectangles. I don't see it as a circle because I see no connection between the end of December and the January of the same year. The next January starts at the beginning of another row entirely unconnected to the previous year.