Ekadashi is called Hari Vasara, "the day of Sri Hari" and refers to the eleventh day of a fortnight in a lunar month. It occurs twice every month, in the bright fortnight and the dark fortnight. Majority of Vaishnavites (Followers of Vaisnavism) observe fast on this particular day. This observance is not merely constituted of avoiding food as fasting can be done any day. There is an astrological phenomenon to this also. We all know that our personality is tremendously influenced by the movement and the gravitational pull of the planets.
Rig Veda refers to Sun as "Atma Karaka" (Significator of Higher Self, Consciousness) identifying with the soul of the universe as well as the soul of an individual. Sun controls the center of our personality. While different planets influence different parts of the body, Sun is capable of influencing the entire being.
Moon has its influence on our mind hence referred as "Mano Karaka" (Significator of Lower Self, Mind). Mind is just matter; it is a subtle portion of the material substance of what we eat. And it is what we eat which forms our Guna (Quality within) namely Sattva, Rajo & Tamo. On new moon day as there is darkness Rajo (Quality of Passion) and Tamo (Quality of Ignorance) are predominantly strong on the mind compared to other days. In contrast on full moon day there is a decrease in the Rajo and Tamo Guna prominence because of the increased illumination. That is why they say moody lighting for the scarcely lit ambiance. Ekadashi has its relevancy to this relationship between moon and our mind.
We all know about certain energy centers called Chakras present in our body. They are not visible physically as they are psycho-physical and psycho-somatic and have their presence in the Astral Body (Mind and Intellect index). Moon which gets the illumination from Sun influence the Chakras which in turn influence on our mind. Hence there is psychic variation experienced by some on the full-moon and new-moon days.
Mind is an internal energy which has no limits. It finds its presence throughout the body if it is searched. At times it can be made to go out of the physical body also and for that greater level of practice is required. But sticking to the topic let us consider that it travels in our body and goes up and down the Chakras. Though it goes to every part of the body it has two places which it can say “My Own Place” that is the Ajna Chakra (between the two eye-brows) the Rational Thinking spot and Anahata Chakra (Heart) the Emotional Thinking spot.
In the fortnightly movement Mind finds itself in its comfortable place in the Ajna Chakra and the Anahata Chakra on Ekadashi (the eleventh day). In Shukla Paksha (waxing moon phase) Mind is in the Ajna Chakra and in Krishna Paksha (waning moon phase) it is in the Anahata Chakra. As these two Chakras are the abode of mind, mind gets focused and connected to Absolute Consciousness in a jiffy at those days.
Our ancients have understood this secret and had taken advantage of these two days to focus the mind on Absolute. Fasting help in keeping body light and stomach free from excessive metabolic activity. We might have experienced that after a heavy meal we tend to feel sleepy because the blood circulation is accelerated towards the digestive system and hence there is deficiency in blood circulation to the head and so we feel drowsiness. With a feel of drowsiness and lethargy, mind fails to concentrate and focus. Fasting once in fifteen days on Ekadashi will bring feel of cheeriness not weariness. Those who are healthy can observe complete fast while those who are sick and weak can avoid rice and cereals and have milk and fruits on that day. This helps in controlling mind and also will power.