A calendar helps when choosing travel dates, but it does not predict an individual day. TAT describes September to December as the rainiest period on the Gulf coast. That is a broad seasonal pattern, not a promise of rain or sunshine during your stay.
Check nearer the outing
Use a fresh forecast and local warnings for a day trip. If heavy rain starts, do not try to complete every planned stop regardless. A short walk near your accommodation can make a better day than a long drive around the island.
Have another option
Do not commit your only free day to several distant places at once. Choose something near the hotel as a fallback and leave room to postpone an outing. Pack belongings and documents so they stay dry if the weather changes.
Include weather in transport choices
With children, luggage or daily errands you would rather not fit around showers, compare a car with a scooter. Having a car still means paying attention to road conditions. Agree the hire period and any extension arrangements in advance rather than assuming extra days will be available.