It would be "du premier septembre", so I would expect this to be pronounced "de l'onze septembre."
Is that correct? Is it only written this way because it's written with numerals?
It is always correct not to elide the word preceding both cardinals and ordinals in French so you can take it as a rule without risk.
Elided form might sometimes be observed but only to match (lazy) spoken French, :
This doesn't apply to un and une when used as an article. In the opposite the elision is mandatory in that case, eg:
This leads to both of these acceptable forms:
There are no elisions with Onze when used in dates even in spoken French, so it is always:
Note that de le might appear when le is a pronoun like in (thanks to Aerovistae to point it):
De le or de les are sometimes used for humorous purpose like in:
It is also sometimes used by mistake before proper nouns while elision should be used:
“Onze” is a weird word. You generally don’t elide the word that precedes it, as if it started with an aspirated h. Thus, you don’t say “*
Because otherwise French would be no fun, there are of course exceptions to the “don’t elide in front of ’onze’” rule. You can (but do not have too) elide “de” and “que” in front of it. All the following sentences are correct.