Well, like everything fantasy, no.. you don't... If you're looking for a science behind it, kind of. If you think about it, if your world rotates on an axis, like ours, if that axis is steady, the equator is the line in which the world is always closest to the sun, and therefore the hottest. there are plenty of ways around this simple explanation.. for instance, who says the middle of your map is the equator? the equator is often located on the middle of world maps, but there is nothing saying that's the case. Many maps, especially older maps, would just focus the centre of the map on where they are, or where they see the focus of the world is (for instance, the capital, or region of the ruling faction). if it's geographical, its a bit harder (as in you have decided the north and south are frozen glaciers, meaning they are most likely the far points from the sun... it'd be interesting to play with things like a non-stable axis rotation, meaning the world doesn't spin, it could wobble, or something else, which would modify your climates.

many people don't think about the geography of a world in to much detail, so you could even just leave it unexplained, and let people come to their own conclusions. it's your world afterall, it obeys your laws... it's only magic if you say it's magic.