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:
- If I was a bird, I would fly to Paris.
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:
- It is Harry’s birthday tomorrow.
- The train leaves at 9am.
However, we can of course use a future simple construction with the modal verb will:
- It will be Harry’s birthday tomorrow.
- The train will leave at 9am.
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.
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.