The Ember Days are the Wednesdays, Fridays and Saturdays following Ash Wednesday, Pentecost, the Feast of the Holy Cross and St Lucy's Day. They are traditionally days of Fasting and Abstinence at the four seasons and ordinations were conferred on the four Ember Saturdays. Whilst His Holiness Pope Leo the Great (440-461) considered the Ember Fasts after Pentecost, Holy Cross and St Lucy to be an Apostolic Tradition, the fourth Fast, after Ash Wednesday, was a later addition, though His Holines Pope Gelasius (492-496) speaks of all four. Unknown in the East, they spread slowly in the West, brought to England by St Augustine, adopted in Gaul by the Merovingas, and observed in Spain in the XIth Century, they were only introduced into Milan by St Charles Borremeo, whose charge of the Diocese began in 1560.
From the Catholic Encyclopædia.
From the Golden Legend.
From the Encyclopædia Brittanica, 11th Edition, 1911.
From an Anglican site:
Here is an odd little mnemonic that was once used to remember when the Ember Days fell:
Sant Crux, Lucia, Cineres, Charismata Dia
Ut sit in angariâ quarta sequens feria
Loosely translated into English:
"Holy Cross, Lucy, Ash Wednesday, Pentecost,
are when the quarter holidays follow."