Some people accept both. Many know of neither. It is named after the two days, Kajeng and Keliwon, and occurs every 15 days. Kajeng Keliwon is a good day for prayers, and many temple anniversary festivals and other religious ceremonies are held on Kajeng Keliwon. But the day is also especially dangerous because evil spirits are about.
Every family makes special offerings to guard against the spirits doing any harm.
This Hindu calendar had its origins in South India during the reign of a ruler by that name. The Saka calendar is a lunar calendar. Each of the 12 lunar months ends on a new moon, called Tilem. The calendar begins the day after the new moon that ends the ninth lunar month - almost always in Gregorian March. New Year's Day, the first day of the 10th lunar month, is called Nyepi and is an important religious day.