Baring-Gould & Fisher (LBS ii, 271) give references to a number of sources which agree on 8 October. They also, however, note others which suggest 9 October, or the second Thursday in October, or 14 March. In Ireland, they say, his feast was February 11, and this is the date that Hugh Thomas, very much a Breconshire man, takes from a martyrology (Thomas oral tale). He goes on, however, to say that because this date often fell in Lent, and could not be ‘so hospitably keept as the People desired’, it was now observed all over the county on the second Thursday in October.
Browne Willis 1733: 180 records a date of October 7 in association with two Cynog dedications – this is intriguing: whilst the 8th and 9th could be reconciled with ‘the second Thursday in October’, the 7th cannot. The same writer gives 8 October at Llangynog MTG (Browne Willis 1733: 220). According to Evans and Francis 1994–5, Cynog is 9 October in the modern Church of Wales prayer book.