Full moon is a lunar phase that occurs when the Moon is on the opposite side of the Earth from the Sun. As seen from Earth, the hemisphere of the Moon that is facing the earth (the near side) is almost fully illuminated by the Sun and appears round.
In lunar calendars (Panchang) in which each month begins on the new moon, the full moon falls on either the 14th or 15th of the lunar month. Because lunar months have a whole number of days, lunar months may be either 29 or 30 days long.
Ekadashi is the eleventh (11th) lunar day (Tithi) of the shukla (Bright) or krishna (Dark) paksha (Fortnight) of every lunar month in the Hindu calendar (Panchang).
In Hinduism it is considered a spiritually beneficial day. Scriptures recommend observing a fast from sunset on the day prior to Ekadashi to after sunrise on the day following Ekadashi.
Two Ekadashis occur in one month according to different positions of moon. One Ekadashi also coincides with the times when the Moon is the closest to Earth (perigee) and the times when it is farthest from the Earth (apogee).
During Ekadashis in the Krishna Paksha, the Moon is at its closest to the Earth and during Ekadashis in the Shukla Paksha at it’s farthest from the Earth.