You will fly in to OAX , Oaxaca International Aiport.
March is the least humid month at about 50% however it is also the second hottest month with average highs of 88 degrees and lows of 53 degrees so be ready for some heat.
It will be traditional formal and cowboy formal attire.
Most guests have rented airbnbs, I suggest checking those out or you can get a hotel near Templo Santo Domingo de Guzman which is centrically located.