Weather can change quickly on the Seychelles. While it is warm all the time, with only small variations in temperature, a raincoat or a pocket umbrella might be nice during a rain shower ;-). If it´s not raining you can also use your umbrella as a sunshade.
We do not recommend a special time of the year as „the best travelling time for the Seychelles“, because it might or might not be true for your stay. Weather is not always following the statistics ;-)
But there are some facts you can read on many different weather sites of the world and decide what fits you best.
Some statistics mixed with some experience from our trips:
Climate on the Seychelles is influenced by the alternation of two winds, the rather humid northwest monsoon from December to March that brings more rain and thunderstorms (but usually also some sun after a few hours of rain) and the dryer southeast monsoon from May to October, where it sure can rain, but it should be less and it is not as humid, The sea breeze makes the trip pleasant, but the sea can be rough at times.
During the months of transition (around April/Mai and October/November, but even that changes slightly) humidity is highest and the air is calm. If you come for diving or snorkelling you might choose these months because no wind usually means no waves and better underwater visibility. On the other hand, if you would like to take a sailing trip, no wind might not satisfy you too much. But as already mentioned, weather doesn´t care about statistics and we have seen quite some strange periods.
continuous rain is unusual, but it sure can happen (most likely in January and February, causing even minor floods)