Ekadasi is a Sanskrit word for number 11. It means the 11th day of each half of the month in the Vedic lunar calendar. So there are two Ekadasi days in each month on the Vedic calendar. One is in the first half of the month whilst the moon is waxing, or growing. This is known as the bright fortnight. The second is in the second half of the month whilst the moon is waning, or shrinking. This is known as the dark fortnight.
Ekadasi has been set aside for us to observe a fast to please Krishna and is a special day for devotees. Srila Prabhupada asked us to fast on Ekadasi by avoiding eating grains and beans, anything made from them or anything that has them in it. This means avoiding bread, pasta, lentils, rice, as well as beans and preparations made from bean flour.