The seasons in Seychelles are defined by the winds. The islands have two seasons based on the dominant wind pattern during those months, namely the Northwest Monsoon and the Southeast Monsoon (also known as the Southeast Trade Winds), from now on NWM and SEM, respectively. The year usually looks like this:
Jan - NWM
Feb - NWM (dies out around late Feb)
Mar -
Apr -
May - SEM (comes in around late May)
Jun - SEM
Jul - SEM
Aug - SEM
Sep - SEM (dies out around mid Sep)
Oct -
Nov - NWM (comes in around mid Nov)
Dec - NWM
NWM brings heavy rain, overcast skies, light to moderate wind, and rough seas on the north and west facing coasts.
SEM brings passing showers, moderately overcast skies (with more clear days than NVM), cool temperatures, strong winds, and rough seas on the south and east facing coasts.
Weather-wise, the best time to come to Seychelles are the months in between NWM and SEM. These are the calm transition months with little wind, blue skies and lots of sunshine (though they can get quite hot). So basically March, April and October.
The border months like February, May, September and November can also work depending on when you are visiting.
And of course, with El Nino this year, and climate change in general, the weather patterns have been getting more unpredictable (just look at the amount of rainy days we are getting now in, what is supposed to be, sunny October), so keep that in mind. But generally that's how the seasons have been.
Hope this helps