What does "Still, you weren't eleven every day" mean in Harry Potter and the Philosopher's Stone? [closed]

The context:

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.

This is a common sentence; there is no complication about it, and moreover it is a pattern for thousands of sentences.

• You don't get married everyday. ( You didn't get married everyday.)
• It's not Christmas everyday. (It wasn't Christmas everyday.)
• You don't drink fifty year old whiskey everyday.

It means that whatever the event that is being mentioned in the sentence, it is something important and rare in someone's life, and because of that there is reason enough to make the most of it or to take special precautions, or show special attention to someone, etc.

When one says something "doesn't happen every day," one means that the event happens rarely, and that one should take notice of it or celebrate it, despite whatever else is going on.

Harry is in a miserable situation with his mild-to-moderately abusive family and doesn't expect much worth celebrating. Still, tomorrow is the only eleventh birthday he'll ever have, and he figures he might as well make the most of it.