Will December 21st really be 3 days of darkness

No, stop thinking about it . Some pastor claimed that the world would end on October of last year. Others said it could have ended in 2000. If you're an atheist, some scientists have already denied all possibilities. If you're a Christian or believe in God you already knows what the bible mentions about the end of the world.
It won't happen, relax!

That's not a "friend", more of a "fried egg for brains".

Did he explain HOW something that has NEVER HAPPENED BEFORE is supposed to happen? I'd be curious, but I'm not brain-dead or in Grade school.

Try to LEARN from this experience, please.

No.
It's a hoax.
NASA never predicted the world coming to an end or three days of blackness, so why would you listen to some random strangers?

Do yourself and he entire world a favor: in broad daylight on Dec 21, walk up to this so-called friend and laugh loudly in his face, attracting maximum attention from anyone nearby so they can join in ridiculing this jackass

No.

Your friend is trying to scare you (and is succeeding) but the sun will not turn off for three days any time in the next five billion years.

This will only happen at night, after you are in bed. The next morning, it'll be nice and light (depending on local weather).