I've done both...spending the last four christmas' in sxm french and dutch side, and tobago too during covid.
Ive also been for the month of jan over the last decade.
Dec is a mashup of events and traffic and large crowds with a great festive spirit. I also paid extra for most everything.
January is much quieter, simpler experience. Kids are back in school and a refined crowd reappears. There's a lull between the first week of jan and say the 3rd week where the crowds and villa selection may be relevant. Service is more relaxed and the days carefree.
I support ur big bday xmas event...as i did the same every five years for my birthdays.. why not try something different?
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