Why is it observed right before Ashadha Shukla Devshayani Ekadashi?

Because Ashadha marks the gateway into Chaturmas (the 4-month cosmic resting period). Cleansing one's body and karmic slate on Yogini Ekadashi prepares the seeker for the intense spiritual austerities of the upcoming monsoon months.

Is this vrat restricted only to those suffering from physical illness?

No. While its curative power for illnesses is legendary, anyone seeking spiritual purity, peace of mind, and the eternal grace of Lord Vishnu can and should observe this vrata.

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Annual

Yogini Ekadashi

The supreme affliction-cleansing Ekadashi observed on Ashadha Krishna Ekadashi before the onset of Chaturmas