 The datu would read the porhalaan to determine which day is considered auspicious or inauspicious to hold a certain ritual.In order to minimize the risk of accidentally selecting an unfavorable day due to errors in calendar management, days are often chosen based on whether the day is able to promise happiness in two months time, probably the current month and the following one.There is often an extra 13th month in the calendar that serves this purpose, originally a Hindu leap year, but in the Batak context, it is used for a different reason.