Why do they call this Friday - "Good" ?
The Friday set aside to remember our Lord’s death, has had a number of different names. “Sorrowful Friday” has been used in Germany, “Long Friday” in Denmark, and “Good Friday” in both Germany and England.

Though the actual “first use” of the expression “Good Friday” is unknown, there is a strong possibility that it may have once been called “God’s Friday.” It was, after all, the day when God set in motion the way of salvation for all people — the sacrifice of His Son for our sins.

For those who enjoy word origins: in Old English the word for “God,” was “God,” a noun. In Old English the word for “good,” was also “god,” an adjective (though perhaps with a slightly different pronunciation?). As modern English evolved it is possible that because of the similar spellings and sounds of the two words — common usage just made the substitution. Fact is that whatever the real reason was, it has been lost in time.