Knitting for the Seasons

Okay... Here's a question for you. When do you switch over to knitting spring/summer sweaters and when do you start on fall/winter sweaters? Maybe it's the fact that it's been in the 70's for the past few days, which is rare for central VA, but I'm really getting the urge to start on my spring sweaters. I have the yarn for several winter sweaters, and I've already started a couple of them. But I'm wondering what's the point of spending the rest of the winter knitting them when by the time I've finished, they'll be too warm to wear. And plus, I won't have any cool new sweaters ready for spring. On the other hand, I don't want to knit with heavy wool yarns in the summer either to have them ready for next fall. What's a girl to do?