While he has no right to dictate when he takes his holiday, he has no need to be thankful he has holiday as it is a statutory right and part of his remuneration. He earned it, it is not a gift.
I got the impression that post was meant more in a 'be thankful you get vacation' because you could be working in a country where you aren't entitled to holiday or something like that, or you might not even have a job to be taking vacation from.