The following is from J. K. Rowling's Harry Potter and the Philosopher's Stone (US title: Harry Potter and the Sorcerer's Stone):
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.
I suspect, it isn't correct and grammar-wise it would be better to use "would be Harry's eleventh birthday". I want to understand, why "was" is used and why "would be" is better to use.
If it was Monday then tomorrow, Tuesday, was Harry's eleventh birthday.
The sentence above looks a bit like a subjunctive conditional (sometimes called a remote conditional or 2nd/3rd conditional).
Here is a genuine subjunctive conditional:
Notice that this conditional, like all subjunctive conditionals, has a past tense modal verb in the main clause—in this case, as is most common, the verb would.
However, the conditional from Harry Potter is not a subjunctive conditional. It is just a normal conditional, but talking about the past. We might think of this as a reported situation or reported thought, a bit like reported speech.
If we travel back to the time of the story, and this thought would look like this:
If it is Monday today then tomorrow, Tuesday, is Harry's eleventh birthday.
if we shift that back to the past again, we will need to change the present form is to past form was. If we do this we get:
If it was Monday today then tomorrow, Tuesday, was Harry's eleventh birthday.
Now, when we talk about forthcoming birthdays, we most usually describe them using the present simple. The reason for this is that they are fixed in the calendar and do not change. We often use the present tense for timetabled events:
However, we can of course use a future simple construction with the modal verb will:
These sentences are perfectly grammatical, they just present the information as a prediction rather than a fixed event.
In the same way, of course, the information in the Original Poster's example, could have been portrayed in a similar way. If we travel back to the time of the story, the thought could have been like this:
If it is Monday today then tomorrow, Tuesday, will be Harry's eleventh birthday.
If we then translate this into the past, we need to change is to was, and will to would (because would is the past tense of will):
If it was Monday today then tomorrow, Tuesday, would be Harry's eleventh birthday.
Conclusion:
The sentence in the Harry Potter novel is completely correct and perfectly grammatical. However, the Original Poster's intuitions are correct that it would be possible to use would be instead of was in the main clause there. That would also have been a fine choice, although the flavour of the sentence might be subtly different.
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.
The sentence
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.
