Purnima fasting might be observed either on Purnima day or on one day before Purnima day i.e. on Chaturdashi day depending on starting time of Purnima Tithi on previous day.
Purnima fasting on Chaturdashi occurs only when Purnima starts during Madhyahna period on previous day. It is believed that if Chaturdashi prevails beyond Madhyahna then it pollutes Purnima Tithi and this Chaturdashi day should not be considered for Purnima fasting even if Purnima prevails during evening. There are no two opinions on this rule and DrikPanchang.com lists Purnima fasting days according to this rule.
28 January (Thursday) Paush Purnima Vrat
27 February (Saturday) Magha Purnima Vrat
28 March (Sunday) Phalguna Purnima Vrat
26 April (Monday) Chaitra Purnima Vrat
26 May (Wednesday) Vaishakha Purnima Vrat
24 June (Thursday) Jyeshtha Purnima Vrat
23 July (Friday) Ashadha Purnima Vrat
22 August (Sunday) Shravana Purnima Vrat
20 September (Monday) Bhadrapada Purnima Vrat
20 October (Wednesday) Ashwin Purnima Vrat
18 November (Thursday) Kartik Purnima Vrat
18 December (Saturday) Margashirsha Purnima Vrat