I am surprised the kids were handing out their invitations themselves. Because, usually, Teachers have etiquette rules on that... meaning: that invitations are given to the Teacher to disperse.... and the Teacher will do so privately and not in an obvious manner, or put it in the child's mailboxes etc. SO that, other kids do not get hurt feelings, if they are not invited. That is normal usual classroom Teacher rules.
Next, well, there was no premeditated 'planning' on the other boy's part or his Mom, that they were going to give out their invitations the SAME day as your son. So that couldn't be helped.
She probably said it is a 'wedding' because, it was an awkward moment and she was caught off guard (as your were) and she didn't know what else to tell you... and didn't want to seem rude or mean.....
Or, she was just lying obviously.
Its not like she KNEW your son's party was on the same day as yours. So the 'rudeness' was not 'planned.'
And its probably awkward for everyone..... and of course they are not telling your son anything either... Because, the other kids' parents probably are telling their kid not to say anything to your son.... so his feelings are not hurt....
Regardless... that the other boy's party is 6 days beforehand or on the same day as your son's party... is well, bad timing for your son.
But, these things do happen. More than you'd think.
And ESPECIALLY if kids are 'allowed' to give out their invitations themselves... in plain sight, of the other kids. This is NOT good.
You just need to explain to your son, without talking 'stink' about the other boy/situation...that sometimes these things happens. My girl is 7... and if that happened, I would simply explain to her... that there is another party that day, and so, there is not a good turnout for her party.
He will be sad, or not.... so, just gauge him.
Since he has 2 attendees to his party... then maybe, you can get them more special goody bags etc., since the budget will be less now. And so maybe you can go more all out for his party. With just 3 kids. Look at it that way.... less is more.
all the best,
Susan