“Spring and fall sunlight contains less ultraviolet light, though, so it makes sense to get more sun in the spring and fall.”
In the reading I’ve done about Vitamin D I’ve never seen that recommendation, including numerous Vitamin D Council newsletters (John Cannell, M.D.). In fact, the recommendations all point to summer sun as the best for Vitamin D stimulation due to the decreased angle of incidence and resultant increase in ultraviolet energy. Of course, I may have missed it...I’d be interested in any links or studies you might have regarding spring and fall being better for Vitamin D generation.
I didn't mean to imply that more D can be created in spring and fall. It's more difficult to get Vitamin D during the spring and fall which is precisely why it's important to spend more time outside during those times of year. In the summer it's very easy to get enough D. For school children a 20 minute recess outside in June would be enough. But it takes longer during spring and fall to get the same amount of D. What I propose is allowing children to have month long spring and fall breaks from school so that they may spend more time outside during those times of year when D is harder to make. And of course skin must be exposed. Temperatures in spring and fall in the middle of the day are often warm enough to permit short sleeve shirts and short pants.
Winter is more difficult, of course, though for me I've created a special sitting area shielded from the wind that allows me to sun some days in winter when the sky is clear.