The Anantacaturdasi-vrata (Bhadrapada sukla caturdasi) seems to be the most important and widely observed. It is also called 'Anantapadmanabha-vrata'. Lord Visnu is believed to have appeared on this day as Padmanabha lying on the couch of Ananta (the thousand-hooded serpent).
Fasting till sunset
Here is a statement by Srila Prabhupada, ISKCON's Founder Acarya, wherein he instructed a vrata should be followed:
Prabhupäda: Yes. Bhaktivinoda Öhäkura 13th, and 14th Ananta Caturdasi and Haridäsa Öhäkura. So, this 14th September should be observed fasting, whole day.
Devotee: Fourteenth is the advent of or the demise of Haridäsa Öhäkura?
Devotee: The death, I mean the disappearance
Prabhupäda: Yes, but not for demise of Haridäsa Öhäkura, but because that is Ananta Caturdaçé Vrata, the, generally the fasting is observed till evening, up to 5 o'clock. The next day, Viçvarüpa Mahotsava, feasting. (11th March 1972 Vrindavana Room Conversation)