Mauritius has just two seasons, a frequently humid, warm summer extending from November to April, and a dry, cooler winter between June and September. If you visit in the winter, it will still be quite pleasant along the coast at around 75 degrees Fahrenheit, but the water may be a bit too chilly for swimming, especially in July and August, although it rarely dips below 64 degrees. In the mountains, you may need a sweater. October and May tend to be the best months, with comfortable temperatures in the mid- to upper 70s, and little rain. December through April is the wetter time of year, with January and February getting the most rainfall. Mid-December through March also brings the possibility of cyclones.
With so many different activities and attractions in Mauritius, the best time to visit really depends on what you hope to do.