Here's how we made our decision:
The winters here are cold and long...and the weeks we are usually the most desperate to escape are mid January to the end of February. I love autumn here as well as Christmas at home and all the preparations. It takes us until mid January to pack everything back up again and get the Christmas stuff put away for another year. My fiance's daughter was writing exams the last week of January. Travel in mid to late Feb was considerably more expensive, so we went with the very end of Jan/beginning of Feb.
We had been to the Mayan Riviera twice before and loved it. The resort we picked was in the price point we wanted ($1500-$1800 pp), was an all inclusive, had lots to offer our guests on site and was easy to take day trips out (e.g., Akumal to swim with the turtles, Tulum to see the ruins). Having a civil (legal) ceremony in Mexico for us was easy peasy. They needed three business days to process the paperwork after we arrived, and we did have to do the bloodwork but that was no biggie. If either of you have been married before or if you'll be changing your name when you get home, there will be an extra step or two.
When we factored in the time the resort needed to process the paperwork after we arrived...when our preferred ceremony/reception site at the resort was available...and when our dream photographer was available...we came up with two possible dates...Feb 2/12 or Feb 3/12. Our TA was really helpful in making the calls back and forth at this point to ensure we were getting exactly what we wanted. Once these things were set in stone, everything else came together just fine.