We observe Ekadasi from sunrise to sunrise

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.

Letter to: Madhudvisa  --  England 30 September, 1969

Ekadashi Update:Putrada Ekdashi

One has to perform devotional service in full tapasya, austerity. One should fast on the two Ekadashi days, which fall on the eleventh day of the waxing and waning moon, and on the birthdays of Lord Krishna, Lord Rama and Caitanya Mahäprabhu. There are many such fasting days. Yogena means “by controlling the senses and mind.” Yoga indriya-saàyamaù. Yogena implies that one is seriously absorbed in the self and is able, by development of knowledge, to understand his constitutional position in relationship with the Superself. In this way one becomes fixed in devotional service, and his faith cannot be shaken by any material allurement. 

(ACBSP. Srimad Bhagavatam 3:27:22. purport excerpt)