It's a pretty logical question when you think about it...The celebration is the triumphant risen Christ, old bean. "Good" Friday is a day of mourning and grief.
When I was a kid, I remember asking why do we call the Friday before Easter "Good" Friday when it was the day Jesus died...it doesn't seem like there was anything "good" about it...
And my kids have asked the same question...there should probably be a better word for it than "Good" Friday...
The answer though is, that you have to look at the larger picture...
It's "good" in the sense that it was the day that our Lord and Savior died on The Cross for our sins...
And without that, one would not have The Resurrection, symbolized by Easter Sunday, (who knows for sure on what dates these events actually took place...the important thing is that they did take place...)
I think I have a pretty expansive and "liberal" view about who qualifies for membership in the "Christian Camp"...
I believe one can have a lot of different opinions on a lot of different subjects, and still qualify...
But if you do not believe in the divinity of Jesus Christ, if you do not believe that Christ Was God Made Man, and you do not believe in The Truth Of The Resurrection...
Then you really cannot properly call yourself a Christian...(of course it's a free country, so you can call yourself a Christian anyway; you can call yourself a root beer float if you want to...)
That certainly doesn't make you a bad person, (in fact you may be a "better" person than many Christians) but if you don't embrace and believe in those two fundamental principles, you are not a Christian...
Believing that Jesus was a "great teacher" or a "really cool dude" does not make you a Christian...
Only accepting The Truth of His divinity and The Truth Of The Resurrection can do that...