why is Good Friday called good?

late 13c., from good in sense of "holy" (e.g. the good book "the Bible," 1896), also especially of holy days or seasons observed by the church (early 15c.); it was also applied to Christmas and Shrove Tuesday.

http://www.etymonline.com/index.php?term=Good+Frid...

Etymology

From good meaning holy

http://en.wiktionary.org/wiki/Good_Friday

Jesus died on a Thursday. Friday counts as the first day, Saturday the second, and Sunday the third day. "Good" Friday comes from a medieval term in which "good" and "holy" are synonyms.

Generally, English speaking countries and the Dutch call Friday of the Holy Week as "Good Friday". Its said that in Early Modern English, "Good" had a meaning in the sense of "Holy".

When Jesus was killed on that day. you answered your own question

It is the day when Jesus Christ died on the cross for out sins. It is the holiest day of the year.

Jesus died for people's sins which is good.

Jesus wanted to die to save the people.

because jesus dies for OUR sins on that day, he did what he needed to do to save us

Because "shi**y Friday" doesn't roll of the toungue as well.

Because He died for our sins, He sacrificed Himself for His people, what He did was Good.