First, you are not going to find, in the genuine texts of J.K. Rowling, anything that is not correct, grammar-wise.
Second, a slightly larger context is:
“It’s Monday,” [Dudley] told his mother. “The Great Humberto’s on
tonight. I want to stay somewhere with a television .”
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.
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.
is "reported speech." It reports Harry's thoughts on hearing the word Monday. Notice the sentence right before it: This reminded Harry of something. Also notice that since this narration is about the past (it is past narration), then to bring Harry's thoughts into the present, you have to do the opposite of backshifting, and that is to, for lack of a better term, 'foreward-shift' the sentence into the present:
If it is Monday — and you can usually count on Dudley to know the days of the week, because of television — then tomorrow, Tuesday, is Harry’s eleventh birthday.
Note, even so, that Harry is not 100% sure that today is Monday; but he assumes that today is Monday because one can usually count on Dudley to know the days of the week. Still, this means that Harry is not absolutely sure that today is Monday.
Thus, this represents a "real conditional." It is not an unreal (or irrealis) conditional; and thus there is no need to use a modal-construction such as would be in the main clause. The only "conditional thing" involved is whether it really is Monday, and this does not represent irrealis (an unreal or contrary-to-fact situation) but uncertainty on behalf of the speaker.
Put it another way, Harry is not saying
If today was/were Monday, but I know it is not, then tomorrow would be Tuesday
(this represents an irrealis or unreal conditional).
So you just stick with the simple present (is) in present narration and the simple past (was) in past narration.
Note that even if Harry is extremely uncertain if today is Monday, you still use the simple present (in present narration) or simple past, if in past narration. For example, let's say Dudley is terrible at remembering days of the week, and we have a sentence such as
If it was Monday — and Harry was far from certain that it was Monday because you could absolutely not count on Dudley to know the days of the week — then tomorrow, Tuesday, was Harry’s eleventh birthday.
Here, the speaker (Harry) is highly uncertain whether it was Monday; but it is still not a case of irrealis or contrary-to-fact situation. Again, we are dealing with uncertainty on behalf of the speaker, and that is not the same as a contrary-to-fact situation; it is not the same as
If it was/were Monday (but it is not; it is really Friday), then tomorrow would be Tuesday.
Here, you use would be (or some other modal construction) in the main clause to indicate that the condition stated in the if-clause is contrary-to-fact/unreal/irrealis. But that is not the case in the sentence found in the book.