Since when does our child's emergency become our emergency? Only if you let it...
Kids are notoriously bad at planning ahead. Heck, they have a hard time seeing past the weekend. But many children are very skilled at trying to get their parent(s) to react to their perceived crisis and pass their anxiety onto their parent.
The best thing you can do is to refuse this "gift" and let your child continue to carry the anxiety.
Parent: "I understand you really want to go to the party, but we had already planned to visit grandma that weekend. Perhaps you can attend the next party if you ask ahead of time."
Now they may not like this, but by doing so you can help your child plan ahead or at least stop you from being a dumping ground for their problem. Those who carry the anxiety about a problem are more motivated to find a solution. Keep the anxiety where it should be in this situation--with them.