The month of Shravan is the fifth month of the Hindu calender beginning from Chaitra, and is the most auspicious month of the Chaturmas. On Purnima or fullmoon day, or during the course of the month the star ‘Shravan’ rules the sky, hence the month is called Shravan. This month is spread out with innumerably religious festivals and ceremonies and almost all the days of this month are auspicious.
Shravan is considered the holiest month of the year. Each monday of this month, known as Shravana Somvar, is a special day in Shiva temples where the dharanatra hangs over the linga or the idol to bathe it with holy water, day and night. Devotees pile the linga high with Bel leaves and flowers and fast till sunset. The nandadeep (24 hour lamp) burns steadily in the temples.
he legend says that when the churning of oceans – Samudra Manthan – took place in the month of Shravan, fourteen different types of rubies came out. Thirteen of these were distributed amongst the demons, except Halahal (poison). Lord Shiva drank the Halahal and stored it in his throat. Hence the name Neelkantha (meaning blue throat) is attributed to Shiva. Read more about Lord Shiva
To reduce the strong effect of poison, Lord Shiva wore the crescent moon on his head. All the Gods, thereafter started offering the Ganges water to Lord Shiva to make lessen the effect of poison.
Since, this happened in the month of Shravana, since then the Shiva devotees offer the Ganges water in this month.
The belief is that in Shravan month, offering milk to Lord Shiva earns a lot of punya.