Can there be a missing on when I say somebody leaves on some date? For example, is it okay if I say:
We'll have to leave September 1st, because we have to be back in school on the 2nd.
instead of:
We'll have to leave on September 1st, because we have to be back in school on the 2nd.
I understand your concern, it's because some native speakers skip certain words while having a normal, fast conversation with others and yes, they are grammatically incorrect.
The sentence "We'll have to leave September 1st" may sound as if you have heard it somewhere, but still it's grammatically wrong. Date, time and venue require a preposition if used after a verb. But in many informal, short conversation people skip these grammar rules, but that doesn't make it correct.
Therefore, the sentence "We'll have to leave on September 1st" is grammatically correct.
Sometimes, native speakers are just lazy ;)
In my opinion, as a Brit who has lived in the US for 20+ years, British English would always include the "on", whereas it is optional in American usage. In both cases there is an unstated object in the sentence, such as "here" or "home".
I think the answer depends on your intended audience: if they are expecting American English you can leave out the "on", but if there are Brits or people expecting British usage, you should probably include it.
