from the GNU version of the Collaborative International Dictionary of English
- adj. days set apart for fasting and prayer in each of the four seasons of the year. The Council of Placentia [A. D. 1095] appointed for ember days the Wednesday, Friday, and Saturday after the first Sunday in Lent, Whitsuntide, the 14th of September, and the 13th of December. The weeks in which these days fall are called ember weeks.
Sorry, no etymologies found.
The author treats first of the Mass (chap. i-xxiii) in relation to its historical development; second, of the liturgy of the ember days