Yeah, my mom had the months of the year in little signs on the wall doing down vertically, and ever since then in my head they've always been vertical.
What an amazing poll, something so deep that I've never given a moment's though to. In my head they're on a mirror clock: January at "12", February at "11", etc. People who see months on a line and not a circle: are you aware that January comes after December consistently?
I wonder if I see them this way because I'm Israeli.
1. There are two "New Years", so the choice of starting point is not unique.
2. I grew up writing in both left-to-right and right-to-left languages, so the choice of direction is not unique.