Ekadashi occurs every fortnight on the eleventh day. Thus there are twenty four ekadashi's a year. Observing upavasa on all these twenty four ekadashi's is a requisite duty of every Vaishnava or devotee of Lord Sri Vishnu. Millions of devotees around the world today follow this custom very strictly. But many who do not observe this all year around, follow at-least two ekadashi's with great devotion. These are Ashadha Ekadashi (occurring in the month Ashadha masa) and Kartika Ekadashi (occurring in the month Kartika masa).