On an island where each day is a perfect day the question of when is the best time to visit the Seychelles is a difficult one to answer. Every day is warm, the temperature never changes more than a degree or two, so I guess it depends on your reasons for visiting. Here are some things to consider...
Winds are seldom really strong with the best winds for kite-surfing and sailing being the May to September trades. Most of the year you'll find enough wind blowing to make sailing enjoyable although, outside of May to September, it is a little weaker than skilled kite-surfers would like.
The only really bad months for wind lovers are April and early October.
All year round is great with the waters always warm and fairly clear. Certain areas experience reduced visibility when the wind gets up, this is particularly the case between May and September, and lots of seaweed floating about at this time of year can make it worse.
All things considered April and October are the best months but you'll enjoy great diving and snorkeling anytime of year.
The May to September trade winds dramatically drop the humidity levels making this the ideal time to explore the islands on foot or by bicycle.
The drier months of June to September are best but avoid beaches on the west coasts of Mahe and Praslin, during these months they tend to have a lot of seaweed washed ashore.