From 11:56 AM until 12:12 PM today, the sun will be high enough in the sky that its powerful actinic rays will reach the earth with enough energy for exposed skin to produce vitamin D.
Of course, the temperature around noon will be about 12℉, so I’m not expecting to have much exposed skin, but it is still nice to know that vitamin D production will be possible.
There might be an unseasonably warm and sunny day at any time this month or next, giving me a chance to make some vitamin D. And soon enough spring will come.
(Ephemeris data valid for where I live. The opening of the vitamin D window will likely be different for you.)