The Saka year starts in the year 78CE. The word "Saka" is associated with a legendary guru: Aji Saka, who came from India to Java and saved the people of Medang Kamulan from the giants who held sway all over Java. After slaying those giants, Aji Saka organized and civilized the region.
Image via Tropenmuseum
The Balinese Saka Calendar defines the official occurrences of Tilem (New Moon) and Purnama (Full Moon) of each month, as well as the waning and waxing moon. It also defines Tithi, which means the quality of the lunar days based on the duration of two faces of moon. Tithi has a very special role to determine their special activities, there are auspicious and inauspicious tithis.
Just like any other lunar calendar, The Balinese Saka Calendar has twelve lunar months, called sasih, with 29 or 30 lunar days each. Each month goes from Tilem to another and is divided into two parts: sukala (waxing moon) and kresna (waning moon).
The Balinese Saka year is started around the spring equinox (vernal equinox) on the day after Tilem nearest to the equinox. This is the first day of Sasih Kadasa (tenth month of Balinese lunar calendar). Balinese celebrates this new year with Nyepi, a day with a complete silence all over the island.
Old Balinese Saka Calendar - Palalintangan; Source: Tropenmuseum
To explain the oddity that the new year falls on the tenth month and not the first, it might have been established based on the observation of a group of stars of Wuluku (the Orion constelation). The first ten months of Balinese agricultural cycle would have been followed by the period of rest. This tenth month signals the upcoming dry season in Bali. Although, there is phenomenon that the mountain areas may still have heavy rains that might cause floods (blabur Kadasa). In the lower areas, people will start replanting the rice for the first crop of the year and tuber plants are harvested. Flowers are blooming during this sasih.
Sasih Kadasa is the most important month of the Saka calendar. Fifteen days after Nyepi on Purnama day, falls Batara Turun Kabeh (all gods are visiting) at the mother temple of Besakih. Many other temples also have their Odalan day (Temple's Anniversary) on this first full moon after Nyepi.