Bulgarian, along with the closely related Macedonian language (collectively forming the East South Slavic languages), has several characteristics that set it apart from all other Slavic languages: changes include the elimination of case declension, the development of a suffixed definite article (see Balkan language area), and the lack of a verb infinitive, but it retains and has further developed the Proto-Slavic verb system. Various evidential verb forms exist to express un witnessed, retold, and doubtful action. Language The curriculum of Bulgarian Educational system focuses on eight main subjects: Bulgarian language and Literature, foreign languages, mathematics, information technologies, social sciences and civics, natural sciences and ecology, music and art, physical education and sports. The school year starts on September 15 and ends in May or June depending on the grade level of the students. Classes meet five days a week and usually take two shifts (morning and afternoon). The school year is divided into two terms with Christmas, Easter and Summer holidays. The grading system is based on numerals, where 6 is the highest and 2 is the lowest grade a student can obtain. Culture A number of ancient civilizations, including the Thracians, Ancient Greeks, Romans, Ostrogoths, Slavs, Varangians and especially Bulgars, have left their mark on the culture, history and heritage of Bulgaria. Because of this Bulgarian nation has one of the richest folk heritage in the world. Thracian artifacts include numerous tombs and golden treasures, while ancient Bulgars have left traces of their heritage in music and early architecture. Thracian rituals such as the Zarezan , Kukeri and Martenitza are to this day kept alive in the modern Bulgarian culture. Education The Bulgarian educational system falls within the continental European tradition. The main types of secondary schools are: general educational, vocational, language schools and foreign schools. Private schools are also being established and they are beginning to compete successfully with state schools. There are fifty-one higher educational institutions in Bulgaria offering degrees at undergraduate and graduate levels. The academic year for most Bulgarian universities begins around October 1 and consists of fall and spring semesters. The academic year covers up to 30 weeks. Classes usually meet once a week for 75 minutes or for two 45-minute periods with a 15-minute break in between.