How do I not give a fuck that my friends make plans without me (and in front of my face)?

I know the answer is get new friends, but we have a group of like 7 friends so if I drop one, it wont matter, i'll still see them.

We have one friend in the group that literally gives 0 fucks whether all 6 of us are somewhere and he's not. I need to ask him his ways lol. But everyone made plans to go to Six Flags today and didn't invite me, even though I was with them last time. Night before Six Flags, it was said at dinner that they would "see each other tomorrow", so I couldn't invite myself.

How do I not care?