The taxes being done now are the taxes for last year. You would need to file them as single, because that is what you were last year.
Your taxes for this year are not filed until next year. If you are still married at the end of this year, or your spouse dies during this year while still married to you, then you file those taxes as married. If you get divorced this year, then you file those taxes as separate.
It does not matter whether you file before or after you get married. Each year, you file taxes for the previous year, and what matters is whether you were married or not when that year ended (on December 31 of that year).