Go here to get general weather data (by month) for SXM: www.weatherbase.com - click your way through to the "Caribbean", then "St. Martin".

If you want more detailed info go to www.underground.com. Type "SXM" in the search box then scroll down to the "History" section and click on the date field, enter your planned dates of travel. You'll find scores of weather charts and statistics for that period. Study the data and you'll know more about the weather on SXM than most locals.