Learn Portuguese (Portugal): Em setembro é outono.

In Portuguese, months of the year like setembro (September) and days of the week are written with lowercase letters unless they are the very first word of a sentence.

Why do we use Em instead of No before the month?

When talking about months, Portuguese uses the simple preposition em (in). We don't add a word for "the" before months, so it doesn't combine to become no (in the).

Where is the word for "it" in é outono?

English needs a placeholder word like "it" to say "it is autumn", but Portuguese doesn't. You just use the verb é (is) all by itself to state the season, weather, or time.

Why do we use é (from the verb ser) rather than a form of estar?

We use the permanent "to be" verb ser (é) to state basic facts like what day, month, or season it is. We generally save estar for temporary conditions, like saying the weather is cold right now.