Well...
1) Sounds like you've done things the 'same way' for a long time. Then your schedule changes things so you can't do what you always do... And are now mad that everyone se didn't change things, and feel hurt & excluded.
I suggest that this is not your family's fault nor intention to exclude you.
Your schedule changed.
You WANT to see your family on different days... But they can't do it.
That's not a slight on you... That's just part of the season. People get BOOKED. There are only certain windows.
2) I make it a point never to get mad at people for forgetting my schedule. I know it, because I live it. But while there are some people (my mum, for example) who can keep 10/20/30 people's schedules in their head (or even on paper)... Most can't. Even though you only have 2 commitments (work & inlaw dinner), it can be difficult to keep straight who has what going on when. Especially when 'normal' has changed and in the back of their heads they're still thinking that you'll be at the time and place you usually are, but won't be.
2.5) While it sounds like you and your inlaws are able to schedule a month out ... Not everyone can. My family plans a month out. My inlaws have rotating work schedules, so the couldn't until a week prior. They are JUST as important (or were, I'm divorced) as my family. But when my mum was asking for everyone's schedules for EXTRA stuff... I couldn't give her ours. We simply didn't know it. No matter how many times asked. So they'd make plans and we'd be pencilled in. THEN once we got our inlaws schedules, we tried to balance things out between families. At first I got a LOT of grief over that. As my siblings have married, and have also gotten crazy schedules... My mum has realized its not just "me". That a lot of people just CAN'T plan out as far as she does. Its not a slight on her, or us not wanting to come, it was about trying to balance.
I suspect that's what happened with your family. A month ago they didn't know what was up with inlaws, work, school, church, etc. Then round 2, most still didn't know. And by the time everyone knew, the only workable time for most is the same time it wouldn't work for you.
3) Its reeeeeally frustrating when the ONLY time other people have free is the ONLY time I don't... But that happens sometimes. Sometimes its me, or my cousin, or my sister, etc. I can't think of any family gathering in my OWN family that everyone is at. I have a large family, though... So the odds are worse. (My bestie only has THREE! and that sometimes even happens to her family.). Think about years when someone aside from you hasn't been there. I'm sure you didn't all conspire ahead of time, or just not care about them. Instead, the time that worked for everyone else didn't work for you.