There are usually 24 Ekadashis in a year. And two Ekadashis are extra in a leap year (Adhika Masa). Ekadashi is considered a day to cleanse the body.
Prathamekadashi also known as Shayani Ekadashi falls on Aashadha Shudha Ekadashi (Aashadha masa, Shukla paksha Ekadashi tithi). Prathamekadashi day is usually observed by complete or partial fast.
It is believed that on this day Vishnu goes to sleep in Ksheersagar and would sleep for four months, which is known as chaturmasa, and wakes up in Kartik masa on Prabodhini / Devotthani Ekadashi.
The greatness of Prathamekadashi was narrated first by Brahma to his son Narada and later by Shri Krishna to King Yudhisthira that can be read in the Bhavishyottara Purana.
Prathamekadashi day is observed by complete or partial fast. The complete fast is with no food at all. Partial fast is by refraining from specific food like rice, grains, and specific vegetables like onions. Instead, only fruits, vegetables, milk, and milk products are consumed. Though the fast is observed as other Ekadashi fasts, the only difference is some kind of sweet is prepared to offer Vishnu. Special Pooja is performed to get blessing from Vishnu. Special dish for naivedya is any kind of Paayasa but varies from home to home.