Why would my DIL get so upset over this?

My son, his wife and 4 kids headed off to an RV Resort for a couple days of swimming and sun.
My 6 year old grandson kept calling it a “holiday” and Mom got really mad and snarled “It’s a vacation!”
He kept doing it and Mom kept yelling until I pointed out that in Europe they call such vacations “holidays.”
I said “Someone may have read him a book by a British author and that’s where he picked it up.”
She settled down after that..but she was SO ANGRY! I, personally, loved it and I shall start calling “camping” as a “holiday!”
Why did this upset her? Why couldn’t she just blow it off?