Using months and dates in French
When you talk about when an event happens, English switches between "in" for months and "on" for specific dates. French has a similar switch, but it uses completely different rules for prepositions and numbers.
In French, you will see month names like janvier and juillet acting a little differently than their English equivalents, especially when it comes to capitalization and the little words that go in front of them.
If you want to say that something happens "in" a certain month, French uses the preposition en followed directly by the month. You do not use an article like "the" here.
However, if you give a specific date, the pattern changes. You drop the en and instead use the definite article le before the number. Notice that you never use a word for "of" in French dates. "The second of April" is simply le 2 avril.
In English, we use ordinal numbers for dates, like "third", "fifth", or "tenth". French makes this much easier. For almost every day of the month, you just use the normal numbers like deux, trois, or dix.
There is only one exception. For the first day of the month, French uses the ordinal word premier. You will often see this written as a number with a small superscript letters (as in 1er), but in speech, you always say premier.
Unlike in English, French months are not capitalized. Words like février, mai, and décembre are always written in lowercase unless they start a sentence.
You must always use le when giving a date inside a sentence. However, if you are just writing the date as a heading at the top of a letter or an assignment, you can sometimes drop the article and just write 4 septembre instead of le 4 septembre. Let us look at one last example to lock in the rule.
To summarize, use en followed by the lowercase month name to say something happens "in" a month. If you are giving a specific date, use le followed by a regular number and the month name, except for the first of the month, which always uses premier.