Parana means breaking the fast. Ekadashi Parana is done after sunrise on next day of Ekadashi fast.
It is needed to do Parana inside Dwadashi Tithi unless Dwadashi is over before sunrise. Not doing Parana within Dwadashi is similar to an offence.
Parana should not be done during Hari Vasara. One should wait for Hari Vasara to get over before breaking the fast. Hari Vasara is first one fourth duration of Dwadashi Tithi.
The most preferred time to break the fast is Pratahkal. One should avoid breaking the fast during Madhyahna.
If due to some reasons one is not able to break the fast during Pratahkal then one should do it after Madhyahna.