The weather in the keys during December is generally in the 70's and 80's during the day and 60's and 70's at night. Very nice weather.
There is no dress code, come as you are, dress up or dress comfortable, it's a beach or island style wedding so suit up or wear your favorite Aloha gear. One caveat, don't out dress the bride. :)
- About 3.8 miles / 7 minutes from Florida Keys/Marathon International Airport (MTH) - About 45 miles / 59 minutes from Key West International Airport (EYW) - About 113 miles / 2 hours 10 minutes from Miami International Airport (MIA) - About 141 miles / 2 hours 30 minutes from Fort Lauderdale Hollywood International Airport (FLL)
If flying into Miami International Airport you may rent a car to have during your trip, the resort will charge a $15 per day parking fee. However, if you do not need a car there are shuttle services available. https://keyscove.com/services/airport-shuttle-marathon/ which has a few options that may fit your needs. (ensure you ask if they pickup and drop off at hotel) or you may need to get a ride from hotel to Marathon Airport. Also, check out https://www.rome2rio.com/map/Miami-Airport-MIA/Faro-Blanco-Resort-Yacht-Club-Marathon. There are a few options as well utilizing public transportation.
While Key West is closer than Miami, the flights will be a bit more costly. However, there are plenty of options to get you to the Faro Blanco, you could rent a car and enjoy the many things the keys have to offer or if coming to the wedding and planning to stay local there are a few ways to get you there and back. Check out https://www.rome2rio.com/map/Key-West-Airport-EYW/Faro-Blanco-Resort-Yacht-Club-Marathon & for the high end attendees https://kw.limo/
Check out our 'things to do' page but the resort has a link with other options as well. https://www.faroblancoresort.com/marathon-experiences/things-to-do-in-marathon