Gabriel: I don’t know about the current customs but some texts are ambivalent about the 14&15. It seems that sometimes it was the 14th and 15th. E.g. the ‘origin story’ in the vinaya says that wanderers meet three times a fortnight, on 8, 14 and 15. (see MN 146, Kd 2.4.2)
@Gabriel, yes the uposatha was both 14th and 15th, but the patimokkha recitation was either 14th or 15th, as presumably they would have kept their patimokkha for the 24hours between 14th and the 15th!
@Brahmali, oh that’s intriguing. I am unable to get my head around lunar calendars, currently. I will take your word for it. 
with metta,