There are two ways to look at a last minute invitation.
One is to take it to heart, to feel overlooked or like you were an afterthought, and then either decline because you’re hurt or go along feeling put out.
The other is simply to be glad to be invited. If you’re free and you’d enjoy it, you say yes and have a lovely time. If you’re not free, decline politely because you already have plans. No drama, no overthinking.
You are assuming they intentionally left you out because they're usually organised, assuming they should know your schedule or that you ought to rearrange things. There's no reason they should know what you're doing, and likewise you can’t keep dates free for events you didn’t know were happening.
Seems much less angsty to not make any of those assumptions, just accept the invitation if it suits you, enjoy your existing plans if it doesn’t, and don't overthink the timing or that it means anything about how wanted you are. Sometimes people just organise things late, and sometimes late places open up for various reasons. Might not be the case here but they did invite you, now it's up to you whether you want to go, are able to go, and will enjoy yourself while you're there.