Ekadashi Tithi is the eleventh date of a lunar month. A lunar month has 30 days and Ekadashi occurs twice in a month. One in Krishna Paksha and one in Shukla Paksha. This Tithi has more importance than other Tithis.
Amavasya Tithi denotes darkness. The Moon is not seen at all and there is total darkness. On this Tithi, Diyas are lighted at various places such as the gate of the house, at crossings, near tanks, and rivers sides.