When the angle of the sun is > 30 degrees, the rays hitting your skin start a process ending with production of endogenous Vitamin D. In Europe, it is possible only between March and September as the sun is high enough. Screenshot from DMinder shows that today, 30 degrees will be from 10am to 2pm, expanding every day as we go closer to Summer Solstice. Getting a tan early in the year enables you to respond to summer sun better. It primes the skin so well for getting the sun, that you don't need sun screen in July, August. Also, gives fuel to the immune system, and it's my ritual since March of 2020. 6 years now.