the extra-month Ekadashis
The Ekadashi of the intercalary (Adhika / Purushottama) month. Dedicated to Lord Vishnu; observed only in a leap-month year.
An adhika masa has its own two Ekadashis, and they have their own names, Padmini in the bright fortnight and Parama in the dark. They only exist in years with an extra month, which is why a normal year has twenty-four Ekadashis and an adhika masa year has twenty-six. Considered especially meritorious precisely because they're rare.
Standard Ekadashi observance, no grains.