When do you use the preposition in or on?

Try this: http://owl.english.purdue.edu/owl/printable/594/

It is a good website. Thanks! How about what I wrote in my question? Is on used in September 10, 2009?

Read up and tell me yourself. But as a general guidline, we say on Thursday, on the fourteenth of May, but in the evening and in September. Remember: day or date, on; month: in.

Wow, consider the following sentence: ” At 7:00 AM on the second Saturday in October of 2009, @unodos asked a question.”

Wow…that is crazy. Gotta love English.
And the use of “gotta” in a sentence about appreciating the language.

One more wrinkle: in the morning, in the afternoon, in the evening, but at night

On the hour, but at one o’clock.

To continue @LostInParadise‘s “wrinkle”: at night, but during the day.

In 2009, and in the fall, but in the fall of 2009. At 1:00 on a Tuesday in the fall of 2009. (And yes, “on September 10, 2009.”)

During the day, but in the daytime.

It isn’t crazy. There’s some logic to it, but mostly it’s idiomatic, meaning “that’s just the way we say it.” Other languages have the same kinds of usages to memorize.

Did you know that in Latin, the word for “in” and “on” is the same word? (It’s “in”, which also means “into”, “at”, and “against” depending on context).

Leave it to English to make things confusing. I have to agree that it’s largely idiomatic. You just have to try it and see if it sounds right. There are no all-encompassing rules; each phrase is dealt with on an individual basis.

It will sound right only to a native speaker, who absorbs these usages with the air he breathes. When learning the language, you simply have to work at mastery. There are plenty of English learners to whom something sounds right that strikes the ears of native speakers with a painful buzz.

It is not just English. English is not just about being confusing. English blends the logics of many languages, and that makes it complex, but why do we take such giddy delight in its quirks? It is not as if other languages had none. Try learning the counting systems in Japanese: one set of numbers for counting days, another for counting people; one for small flat things, another for round things, another for animals. Or something like that. I don’t remember them all and never did find out what you do if you have a small flat round thing. And yet there is no plural noun: one pencil, two pencil, and you say “there are pencil in the box” with no distinction between one and many.

@Jeruba

I’m familiar with Japanese; I am learning it. My comparison was strictly between Latin and English because those are the only two languages that I “know”. I’ve only begun in Japanese and Russian.

Of course I know that all languages have quirks. But most people who only speak English are only familiar with English’s quirks. Latin has a dative case primarily for the indirect object, but sometimes it is used as a possessive just like the genitive. I remember learning that and thinking “wtf?!”.

And it is a fact that English has the most inconsistent pronunciation out of any other major language.

Make it into a sentence. Then you’ll know if it’s correct.
“I bought that table in July 19, 2001.”
“I bought that table on July 19, 2001.”
It seems a bit primitive, but I find it helps almost every time. I only have to go to the official rule book once in a while.

Also, if you are only using the month in the date, it changes.
“Tom broke 100 on July.”
“Tom broke 100 in July.”

Lemon squeezy.