Sadly I know the feels...

Some friends you got. People don't usually forget. Maybe they just don't like you?

Ouch. That's got to hurt.

If he's yore best friend the he assumed that you don't need inviting and that you were going to come anyway. So all you needed to do was show up to where they were. I assumed they planned all this stuff while you were around or were at least in the loop?

How can you be best friends if they still haven't realised you weren't there? Unless they do know and want you to take a hint. Time to make some changes.

Maybe it wasn't an invite thing? An open party you just didn't hear about

Maybe because you've been friends for so long, they felt that they didn't need to invite you for you to show up?

Wow some "friends" first they don't even invite you and second by your best friend showing you the pictures it's like them rubbing it in your face, it may not be deliberate but they still could of been a little more courteous of your feelings, I'm sorry your "friends" didn't invite you OP maybe this year you will meet new friends so that next new years they can invite you to spend it with them.

Ask him why.

Just because they are your friends doesn't assume they should or would invite you. I have friends that were invited to a party that can't handle themselves when they drink so I didn't go. Your friends aren't required to invite you suck it up.