It is the eleventh lunar day of the bright fortnight of hindu month of ashadha. This day is also known as Shayani Ekadashi and Maha-Ekadashi.
According to the Purans, from this day onwards Lord Vishnu takes rest in Ksheer Sagar (ocean). So, naturally all the oceans, rivers and ponds are believed to attain divinity due to the presence of Lord Vishnu in Ksheer Sagar during this period. This is one of the main reasons why great importance is given to taking holy bath during this period.
*Information gathered from multiple sources on Google
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