This is the 155th most frequent Russian word.
сентября
"Сентября" is the genitive form of "сентябрь," meaning "September" in English.
Here, "сентябр" is used in the prepositional case to denote 'in September.'
В сентябре погода становится прохладнее.
In September, the weather becomes cooler.
This example shows "сентября" in the genitive case to specify a date.
Мой друг приедет пятого сентября.
My friend will arrive on the fifth of September.
This sentence uses "сентября" in the genitive case to indicate a specific date: "the first of September."