How do I tell my child that we can't attended another kid's birthday party.

So my daughter came home with a birthday party invitation yesterday from another kid in her class. The party is today, so very short notice. We already have plans for this weekend and are going out to buy a birthday gift and then sitting at a party with people I don't know is not on the list. I know that sounds mean of me, but I have attended a lot of kid parties in the past. Some great and some not so great. Of course, my daughter is extremely upset that she can't go. She won't stop crying or pouting.

So, has anyone had this problem? How do you tell your kid they can't go without upsetting them? It's too late for me now but I would like to know if this happens again.

Edit... thank you all for the replies. I think I should mention that this was the first time I have said no to a birthday party invite. I always take my kids to every invite that they receive. Last year, I went to seven birthdays in the month of October. I have to admit this gets to be too much. So I might sound a little bitter in my post. But I really have plans this weekend to spend time with family that is coming 16 hours out of state to see us, so it would be hard to drop by a birthday party.

This birthday party invite I received didn't even have my daughter name on it, and I have never heard my daughter mention this kid before. She keeps forgetting the kid's name when I ask. Plus, the main reason she seems upset about missing this party is missing out on birthday cake. It just makes me wonder if my daughter actually wanted to celebrate with the birthday kid or did she just want cake?

Edit again... my daughter is doing okay. We got her helping to make cupcakes for the family, and she is back to her sunshine self. I got her to draw a picture/birthday card, and I think I will get some small gift like glitter pencils or cool erasers to give the birthday kid on Monday.