The bloody flesh our only food:
In spite of which we like to think
That we are sound, substantial flesh and blood-
Again, in spite of that, we call this Friday good.
-T.S. Eliot, Burnt Norton
According to the Oxford English Dictionary, the adjective “good” traditionally "designates a day on (or sometimes a season in) which religious observance is held".
The OED states that "good" in this context refers to "a day or season observed as holy by the church", hence the greeting "good tide" at Christmas or on Shrove Tuesday. In addition to Good Friday, there is also a less well-known Good Wednesday, namely the Wednesday before Easter.
The earliest known use of "guode friday" is found in The South English Legendary, a text from around 1290
But in German-speaking countries, Good Friday is generally referred to as Karfreitag (Kar from Old High German kara‚ "bewail", "grieve"‚ "mourn", Freitag for "Friday"): Mourning Friday. The Kar prefix is an ancestor of the English word care in the sense of cares and woes; it meant mourning. The day is also known as Stiller Freitag ("Silent Friday") and Hoher Freitag ("High Friday, Holy Friday").
The Catholic Encyclopedia, first published in 1907, states that the term's origins are not clear. It says some sources see its origins in the term "God's Friday" or Gottes Freitag, while others maintain that it is from the German Gute Freitag. It notes that the day was called Long Friday by the Anglo-Saxons.
At an earlier time it was designated by "Pascha of the crucifixion," in distinction from "Pascha of the resurrection," the Easter festival ( Augustine, De trinitate, in MPL, xlii. 894).