Ekadashi is the eleventh day (tithi) of each lunar fortnight, so it comes about twice a month. It is especially dear to devotees of Bhagwan Vishnu, and each Ekadashi of the year has its own name and story.
Many families observe Ekadashi with a fast. Some avoid grains and beans for the day, some take only fruit and milk, and some keep a stricter fast. The right practice depends on your family tradition, your health and your age, so follow what your elders and family priest advise.
Common ways to mark the day include reading or listening to the Vishnu Sahasranama, lighting a diya, visiting a Vishnu or Krishna temple, and giving food or support to those in need. The fast is traditionally broken the next morning, within the time shown in the Panchang.
Ekadashi dates change every month. You can check them in the daily Panchang and set reminders in the Aaradhya app so you never miss one.