Ekadasi is the eleventh lunar day (Tithi) of the shukla (bright) or krishna (dark) paksha (fortnight) respectively, of every lunar month in the Hindu calendar (Panchang). In Hinduism and Jainism, it is considered spiritually beneficial day. Scriptures recommend to observe an (ideally waterless) fast from sunset on the day prior to ekadasi until 48 minutes after sunrise on the day following ekadasi.
Ekadasi is a Sanskrit word, which means 'the eleventh'. It refers to the eleventh day of a fortnight belonging to a lunar month. There are two fortnights in a lunar month—the bright and the dark. So, Ekadasi occurs twice in a month, in the bright fortnight and the dark fortnight.
The special feature of Ekadasi, as most people know it, is a fast, abstinence from food. This is how it is usually understood. In fact, the fast is only a practical expression and a symbol of something else that we are expected to do, which is of special significance to our personality.
We belong to the solar system — a huge family of which the sun is the head and the planets are the members. The sun guides the activities of this family including us. We are involved in the laws operating in this system. This is used in astrology. Astronomy studies the movements of planets and stars and astrology the effects on the contents of the system. The Ekadasi observance is an astrological phenomenon and it is observed due to this relation we have with some of the planets in the system.
The sun is said to influence the centre of our personality; hence the sun is called Atmakaraka, the soul-influencer of the human body (Sanskrit 'karaka': doer, manipulator, director). In the Rig_Veda the sun is identified with the soul of the universe as well as the soul of the individual. While the different limbs of our body and different parts of our system are supposed to be influenced by different planets, the sun is capable of influencing the entire being. If there is no sun, we feel the difference - our digestion becomes sluggish and our psychical state is also influenced.
The moon is supposed to influence the mind. The mind is also made up of material substance. It is the subtle portion of the material substance of our food. The subtle essence of the food, not only directly taken through the mouth but through all senses, contribute to the make up of the mind or the mind-stuff. Mind is made up of everything that we take. So, matter influences matter. Planets are not spiritual bodies, and yet they influence the mind. The mind's presiding deity is moon. Ekadasi is particularly relevant to this relation of moon and mind.
Another important aspect is the seat of the mind which is also twofold. You may be living in many houses, of which one or two are your own. Svasthana means 'one's own place.' Two Chakras belong to the mind - Ajna in the waking state and Anahata in deep sleep.
If the mind concentrates in the brain, it is active and one cannot sleep because it refuses to go down. If the mind is midway between Ajna and Anahata, it is dream state. In other Chakras it is extrovert. In the two fortnights it finds itself at the Ajna Chakra and the Anahata Chakra on the eleventh day. Since these two Chakras are its own abode, the mind is at home here, i.e. it gets concentrated and collected easily. This has been the experience of ancients and this has to be taken advantage of by sadhakas. Ekadasi day in both fortnights is the occasion when the mind finds itself in its place — in the bright fortnight in the Ajna Chakra, and in dark fortnight in the Anahata Chakra. Sadhakas and Yogis take advantage of these two days and try to practise deep meditation. Vaishnavas treat Ekadasi as a very holy day and also observe a fast on that day.
Fast and meditation - what connection have they? There is really no intrinsic connection between fast and meditation, but there is some advantage in keeping the body light and the stomach free from excessive metabolic function. When the stomach digests the blood circulation is accelerated towards the digestive organs and blood flow to the head gets decreased. One feels sleepy and the thinking practically ceases. Hence, there is no advantage in burdening the physical system on days when one wants to practice Yoga.
The energy of the whole system gets distributed equally if a particular limb is not given any inordinate work. In fasting the energy is equally distributed as there is no digestion. Fast is supposed to cause buoyancy of feeling and not fatigue. So people who are sick and cannot observe a total fast take milk and fruits, etc. People who are perfectly healthy and are confident, observe a complete fast. This helps in control of mind and will.
Apart from all these, there is a necessity to give the physiological system some rest once a while. It may be overworked due to a little overeating or indiscrimination in diet. These irregularities get rectified in one day. Thus the observance of Ekadasi has many advantages — physical, astral, spiritual — and because this day has connection with the relation of the mind with its abode together with the moon, one feels helped in meditation and contemplation.