The Barddas of Iolo Morganwg, Vol. I., ed. by J. Williams Ab Ithel, , at sacred-texts.com
Prydain, the son of Aedd Mawr, was born on the morning of Alban Eilir, that is, the ninth day of March, according to the computation of the Romans; therefore the beginning of the year was fixed on his day, as a day of feast and institution, as it is still to this day. In old times it was called the first day of Cyntevin, 10 the end of Cyntevin being the feast of Alban Hevin 11 From thence to Alban Elved 12 it was called Summer, and also Harvest; from Alban Elved to Alban Arthan 13 it was called Autumn; and from thence to Alban Eilir, 14 Winter.
p. 406 p. 407
According to the computation of the Church and the Courts of Law, Alban Eilir was said to be the tenth of March, but in the chronology of the Cymry and the Romans it was said to be the ninth of March.
Before the time of Prydain, the son of Aedd the Great, the beginning of the year was fixed upon the day of Alban Arthan, which was the ninth of the month of December, according to the chronology of the Cymry. (See John Jones's Almanack, 1752.)
405:10 p. 405 Cyntefin, (cynt-hefin,) the commencement of summer.
The calends of winter, rough is the weather,
Unlike the beginning of summer.--Ll. Hen.
405:11 The summer solstice.
405:12 The autumnal equinox.
405:13 The winter solstice.
405:14 The vernal equinox.