To determine when you should take a trip to the Bahamas will depend on what your looking to get out of your vacation.
If your younger (20+) and are looking to enjoy the beach as well as the night life the best time to come is when there are a lot of people visiting the Bahamas, which is usually March (spring break for the United States) or February (spring break for Canada), otherwise December, and June- mid August are also good times.
If you are a bit older (35+) and are looking for a vacation to relax and enjoy the company of a significant other, or family, then I would suggest coming to the Bahamas at any time but spring break, especially if your not into the night life so much. If you are into the night life it would be best to come during the spring break months outlined above. If you plan on taking advantage of the hotels restaurants and amenities at night then the spring breakers will not interfere with your plans for relaxing.