How the date is fixed
Every Monday in the North Indian (purnimanta) month of Shravana. The purnimanta month begins the day after the full moon; the amanta month of Maharashtra and Gujarat begins about two weeks later.
About
Shravan Somvar is not a single day but every Monday that falls within the month of Shravana, usually four and occasionally five. North India counts the month by the purnimanta rule, so the Mondays begin the day after Ashadha Purnima and end on Shravana Purnima (Raksha Bandhan); Maharashtra and Gujarat count from the Ashadha new moon, so their series starts about two weeks later. The engine uses the purnimanta span, which is the one printed in Delhi almanacs. The Kanwar pilgrimage to Haridwar and the Jyotirlinga shrines takes place in the same weeks.
How it is observed
A fast on each Monday of the month and offerings of water, milk and bilva leaves at a Shiva shrine.