The Origins of the feria
The ecclesiastical usage which identifies the days of the week as feria is a rather curious one. Originally, it was Sunday that was the feria (or feast day) instituted for worship by the Church. The days of the week were not 'feast days' at all.
According to one theory, the change originated with the week of Easter, a week when every day was a feast day. Easter Sunday, the Lord's Day, was followed by a second feast day (Easter Monday), a third feast day (Tuesday), and so on. Eventually, this usage was generalised to the other, lesser weeks of the year and the word feria came to be applied only to the weekdays, completely losing the meaning of 'feast day'.
Another theory traces the counting system back to the Jews, who habitually counted the days from the Sabbath. (More detailed information on the feria can be found at the Catholic Encyclopedia site.)