Monday. This reminded Harry of something. If it was Monday—and you could usually count on Dudley to know the days of the week, because of television—then tomorrow, Tuesday, was Harry’s eleventh birthday. Of course, his birthdays were never exactly fun — last year, the Dursleys had given him a coat hanger and a pair of Uncle Vernon’s old socks. Still, you weren’t eleven every day.
— Harry Potter and the Philosopher's Stone by J. K. Rowling
I am wondering if this sentence is grammatical and correct to use here. If it is, please explain its meaning.
I guess Harry had never had a pleasant birthday since he was a baby, but that the eleventh was not to be like past ones.