Sep. is a short written form of September, the ninth month of the year. You will see it on calendars, school notices, forms, and in dates such as Sep. 21. It belongs to writing, not to speech: when you read it out loud, you say “September,” not “sep.” Some people write Sept. with a t; that is the same month, not a different one. Do not treat the letters as a word of their own.

九月（書面縮寫）