Carnival of Mahón, in February, is certainly of interest.
The island's most important "Fiesta" is San Juan, on June, 23 and 24, celebrated almost without changes for 600 years. It's climax is the so-called "Jaleo", a spectacular performance with horses and medieval dresses. Also a pilgrimship forms part of this festivity.
On June, 29 Verbenas de San Juan, with regional dances and music, take place.
The "Fiestas" of San Martin in Mercadal, on the third Sunday in July, and of Nuestra Señora de Gracia, on September, 8 in Mahón, are both of medieval tradition. During the latter you can see another "Jaleo".