The Ember Days are four sets of three days (roughly equidistant in the year) set aside for fasting by the Western Christian calendar. They were originally also the only days in which clergy could be ordained. They are the in weeks between the third and fourth Sundays of Advent, the first and second Sundays of Lent, the week between Pentecost and Trinity Sunday, and the week after Holy Cross Day. They are on the Wednesday, Friday and Saturday of their respective weeks.
Their origins are almost certainly pre-Christians (since the East has nothing like them); they happen to roughly correlate to the Celtic festivals of Imbolc, Beltane, Lughnasadh and Samhain. One Ember Week occurs in each of the four seasons; the word ember derives from the Anglo-Saxon ymbren, a circuit or revolution, and clearly relates here to the annual cycle of the year.
They were taken off the official church calendar with the reforms of Vatican II, their observance left to the discretion of individual bishops.