Legend believe 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.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. It is considered highly auspicious to wear a rudraksha in Shravan month. As, Mondays or Somvars of Shravan month are specially observed with austerity. All Mondays are devoted to the worship of Shiva as this day is sacred to Lord Shiva. No other Mondays of other months are so greatly honoured.The belief is that in Shravan month, offering milk to Lord Shiva earns a lot of punya.