>> For Christians, it is good Friday.
Not all Christians -- mostly only Catholics (and those Protestant denominations whom don't exactly "protest" the practices of the Catholic church very well). Today does have special meaning for me, though (as does every Friday), for today is the Bible preparation day for the Bible Sabbath (which begins at sundown this evening).
As for the day Christ was Crucified, this still isn't it. The timing of God's Calendar is that the Day of Passover (on which Jesus was Crucified) is the 14th day of the first month of each new year. Each year begins with the new cycle of the sun (what is referred to today as the spring equinox). Each month begins with the new cycle of the moon (which is referred to today as the observable new moon -- as opposed to the astronomical new moon). Since, from Creation, God provided the sun, moon, and stars as signs for His Feasts (Gen 1:14), then we must observe the sign *before* the event. Thus, the Day of Passover is the 14th day *after* the first new moon which occurs on or *after* the spring equinox.
>> ...we should have celebrated it Wednesday.Friday night
>> to Sunday morning is not 3 days and 3 nights.
You err in your understanding. Jesus never said He would be in the grave for 3 days and 3 nights. That phrase is only used ONE time (figuratively being "in the heart of the earth"; Mat. 12:40): compared to the phrase "the third day" -- which is used at least 14 times in reference to His literal Resurrection. Also:
● The Bible explicitly states He was Crucified on the "preparation day" (Mat. 27:62; Mar. 15:42; Luke 23:54; John 19:31) which the Jews *never* use this phrase for *any* day except Friday -- the day before the weekly Sabbath (Saturday).
● Even if you don't accept that "preparation day" only applies to Friday, the Bible also states that the next day was a "high sabbath" (John 19:31). This phrase refers to the fact that the weekly sabbath coincided with an annual sabbath that year. From Leviticus 23, we know that the day after Passover is just such an annual sabbath (the first day of the Feast of Unleavened Bread). If, as you say, He had been crucified on Wednesday, then Thursday would have been the first day of the Feast of Unleavened Bread and neither Thursday nor Saturday would have been a "high sabbath."