Regarding you first question, we observe Ekadasi from sunrise to sunrise. The 12 midnight is western astronomical calculation, but the Vedic astronomical calculation begins either from the sunrise or the moonrise. Generally it is sunrise. Our calculation is like this: when the sunrise is there, Ekadasi tithi (date) must be there. If Ekadasi tithi is not in the sunrise and the tithi begins, say after a few minutes after the sunrise, then we accept that day as previous to Ekadasi. All our ceremonies are calculated in that way. This means we must see the tithi during sunrise. Therefore, sometimes our dates of ceremonies do not exactly coincide like the western calculations. Just like Christmas Day they have fixed up on the 25th December, but our Janmastami tithi is not fixed up like that. My birthday is on the 1st September, 1896, but this year the tithi of my birthday was fixed for the 4th September. So it is very difficult to calculate, therefore we have to take help from the Indian expert almanac astronomers.
( Śrīla Prabhupāda's Letter to Madhudvisa 30th September 1969. England.)
One such occasion for remembrance is Ekädaçé, which comes twice a month on the eleventh day of the moon, both waning and waxing. On that day all the devotees remain fasting throughout the night and continuously chant the glories of the Lord.( Nectar of Devotion chapter 26.)
Just like... The, everything is described in the Bhagavad-gétä. Just like in hospital. In hospital there are many patients, they are not eating anything on the Ekädaçé day. Does it mean that is observing Ekädaçé? He is simply hankering after, “When I shall eat, when I shall eat, when I shall eat?.” But these students, they voluntarily don’t eat anything. We, we don’t say that you don’t eat anything. Some fruits, some flowers. That’s all. So paraà dåñövä nivartate. Just like a child. He has got in his hand something he’s eating. and if you give him better thing, he will throw away the inferior thing and will take that better thing. So here is Kåñëa consciousness, this better engagement, better life, better philosophy, better consciousness, everything better. Therefore they can give up the sinful activities of life and that will promote to Kåñëa consciousness.
The Ekädaçé, why we observe? To become pure.( Srimad Bhagavatam lecture 7:9:8. 21st October 1968. Seatle.)