The FAFSA® wants you to answer based on your current marital status. This means that:
If you are engaged to be married (with a future wedding/marriage date), your current marital status is single. Answer that you are single. You should then not be asked about the month/year you will be married.
If you will soon be divorced (for example, you're just waiting on the paperwork to clear), your current marital status is either: married (if you're still living together) OR separated (only if you're living apart).
Note that you will need to complete your finances section based on your current marital status. In other words:
If your current status is single, you will not need to report the finances of your fiancé.
If your current status is married, you will need to include your spouse's finances on the FAFSA® (even if you are soon to be divorced). However, you can later submit an appeal letter to your college explaining that your 2023 finances (tax forms) are not representative of your finances as a single person in the coming year.