Today it is a known fact that 80% of the India society is Vitamin D deficient. It was probably known to our Rishi Munis and they created rituals so that everybody gets enough Vitamin D from sunlight. Shahi snan in sunlight are mentioned in our Vedic literature.
1. The month of Magha, Vaishakha and Kartik are considered as months for Shahi Snans where one is supposed to worship sun early in the morning and eat calcium rich food whether it is Urad Ki Daal or sesame seeds.
2. The Chhat pooja which takes place immediately after Diwali is also linked to sun worship.
3. The Marghshirsha month immediately after the month of Kartik also involves worshipping sun.
4. Karkit purnima and Vaishakh purnima are especially known for sun worshiping.
Vitamin D deficiency is linked with metabolic syndrome, heart diseases and also with fertility. All these months when sun is worshipped are also the months of increased fertility. After chaturmaas is over, Tulsi vivah starts the marriage season during which one worships both Tulsi and Amla. Both of them are known to increase the quantity of semen and increased ovulation.
Probably, the age old treatment of infertility was to acquire vitamin D, taking calcium rich food and consume both tulsi and amla on regular basis.
The current vitamin D mantra is that 40 days in a year for atleast 40 minutes, one should expose 40% of the body to the sunlight either after sunrise or just before sunset.