How can one have pure Ekadasi? Also why parana time on next day is important?

Answer by HH Bhakti Vikāśa Swami:

I’m not sure what this question means. I presume that by pure ekādaśī is meant śuddha ekādaśī.

Śuddha ekādaśī is a term that is seen in our Vaiṣṇava calendar. Śuddha means pure. 
Here pure means that it’s not mixed with the proceeding tithi or lunar, solar day. The proceeding day is called daśamī, and is not mixed with the following tithi ekādaśī. And therefore it is suitable for fasting. 

If ekādaśī is mixed, in other words, if the time of daśamī overlaps with that of ekādaśī, or if the time of dvādaśī is overlapsed by ekādaśī, then the ekādaśī is not suitable for fasting, and then the fasting should be observed on the next day, dvādaśī.

That ekādaśī which is unmixed is called śuddha or pure. Pure in the sense that it is not mixed with other days.
Now, exactly the meaning of all this is quite complex because these days are tithis or days which are not calculated as the solar day is, by the rising of the Sun and the setting of the Sun, but by the movements of the Moon, which are much more irregular.

And then, another question connected with this is: Why is parāyaṇa on the next day important?

Actually parāyaṇa means something else. Parāyaṇa means attached to, as in Nārāyaṇa-parāyaṇa, a person who is attached to Nārāyaṇa.

Or another meaning is non stop recitation of a text from the beginning until the end, such as a Bhagavad-gītā parāyaṇa.

The actual word is pāraṇā, and this reffers to the time in which one should break the fast. That comes on dvādaśī.

Or one is to fast on Mahā-dvādaśī because the previous ekādaśī is not śuddha, then one is to break fast on trayodaśī.

The only information I could find about this is in Hari-bhakti-vilāsa, in which is stated that one should honor the dvādaśī by breaking the fast in the right time.

So, just as the ekādaśī should be honored by fasting, so dvādaśī should be honored by breaking the fast in the proper time.

Traditionally Vaiṣṇavas would stay up all night, fasting and hearing, and chanting about Viṣṇu, and would break the fast next day.

We find in Bhagavad-gītā that Kṛṣṇa recommends not too much austerity. One should have a balanced approach.

nāty-aśnatas tu yogo ’sti
na caikāntam anaśnataḥ
na cāti-svapna-śīlasya
jāgrato naiva cārjuna

There is no possibility of one’s becoming a yogī, O Arjuna, if one eats too much or eats too little, sleeps too much or does not sleep enough. (Bg 6.16)

yuktāhāra-vihārasya
yukta-ceṣṭasya karmasu
yukta-svapnāvabodhasya
yogo bhavati duḥkha-hā

He who is regulated in his habits of eating, sleeping, recreation and work can mitigate all material pains by practicing the yoga system. (Bg 6.17)

So, that may be a reason why the time is given. That austerity is particularly meant for ekādaśī, and is not to be extended further. One honors Kṛṣṇa prasādam the next morning at the prescribed time.

Hare Kṛṣṇa!