Ember day any of a number of days reserved for fasting and prayer in the Western Christian Church. Ember days traditionally comprise the Wednesday, Friday, and Saturday following St Lucy's Day (13 December), the first Sunday in Lent, Pentecost (Whitsun), and Holy Cross Day (14 September), though other days are observed locally. They date back at least to 5th-century Rome, probably originating in agricultural festivals, though they have long been associated with ordinations.

Ember is recorded from Old English (in form ymbren), perhaps an alteration of ymbryne ‘period’, from ymb ‘about’ + ryne ‘course’, perhaps influenced in part by ecclesiastical Latin quatuor tempera ‘four periods’ (on which the equivalent German Quatember is based).

Ember Day (eccl.) any of the days of fasting and prayer (Wednesday, Friday, Saturday of the same week) occurring at the four seasons (ecclL. quatuor tempora) in the year at which ordinations take place. Late OE. ymbrendagas pl., beside ymbrenwice (-week), -fæsten (-fast); the first el. (OE. ymbren sg. and pl.) may be an alt. of ymbryne period, revolution of time, f. ymb about, around + ryne course (f. *run- RUN); but the possibility that it is based partly on quatuor tempora is suggested by the form of G. quatember.