Ember days are days of prayer, fasting and ordination. It is a time to pray for bishops, priests and deacons, and - more recently - all Christians in their vocation. Some churches make it a common practice - it used to be a universal practice - to ordain persons to the clergy on those days.
The Ember Days occur on the Wednesday, Friday and Saturday of what I will call the "Ember Week." There are four Ember Weeks per year. They are:
Some claim that the name may come from the Anglo-Saxon ymb-ren - a circuit or revolution; something that recurs. And some claim that the word comes from the German quatember, which is a corruption of the Latin quatuor tempora - "Four Times."
The following mnemonic was once used to remember when the Ember Days fell:
Sant Crux, Lucia, Cineres, Charismata Dia
Ut sit in angariâ quarta sequens feria.
Translated into English:
"Holy Cross, Lucy, Ash Wednesday, Pentecost (lit. "Day of Chrismation"),
are when the quarter holidays follow."
I've taken the liberty of translating more loosely for the purpose of rhyme:
"Holy Cross, Lucy, Ashes, Day of the Anointed,
are when the quarter days are appointed."
- Carl Fortunato