Tricky question... the dates of most of the festivals change from year to year as well as the public holidays related to some of the festivals (the government needs to please all religious and ethnic groups however if every festival was a public holiday we wouldn't work at all... ).
By end of November / early December the festivals for the next year are known and I always rely on calendars bought in Mauritius.
There had once been a fantastic website http://www.holidayfestivals.com
giving these details for every country including Mauritius, but somehow it disappeared...
Of course there also more local festivals which you will never find on any listing... and the Mauritian way is more or less to announce them one week in advance...
In August there will be 'La Vierge' on 15.8. celebrating the birth of Holy Mary, mother of Jesus and 'Rakhi' which is a Hindou festival celebrated within the families only, not publicly.
Mme. Tortue (Mrs. Turtle)