There are three kinds of calendar system which is mostly used: based on solar system and lunar system and combination of solar and lunar system. Solar system is based on how many days the earth rotates around the sun; so 1 year has an average 365 days. The lunar system based on the time taken by the moon to rotate around the earth; which is about 29 days + 12 hours + 44 minutes and 12 time of this equals with 1 year of lunar based calendar system.
Bali uses yet another calendar system. Bali uses wuku (week), sasih (moon) and wara / wewaran. There are 30 wuku, 12 sasih, and 10 wewaran. Wewaran consist of 10 groups of the day. One cycle of Bali calendar called 1 oton is 210 days (wuku x sapta wara or 30 x 7 = 210).
Combination of wuku, sasih and wewaran makes each days has such unique meaning. One particular day has more positive energy for doing such things / work. Another particular day has less negative energy and even there is some day when prohibit to do some particular things such as getting married etc.