Each year, between mid-March and mid-April, the people of Bali celebrate their traditional new year, know as Nyepi. The day itself falls on the first new moon after mid-March. Preparations begin a month or so before the festival, with the construction of large bamboo and papier-mâché demonic effigies called ogoh-ogohs by each village. These representations of evil spirits are paraded through the village, accompanied by a cacophony of gongs and cymbals, on the eve of Nyepi, called Tawur Agung Kesanga. On reaching the main square or intersection, the ogoh-ogohs are set on fire. The noise, music and flames are supposed to chase away the demons for another year.
Nyepi itself is a day of total silence. No work is done, no fires may be lit, and no one should be seen outside (I'm told there should be no lovemaking on this day as well). Most of these rules even apply to tourists, so you need to be aware of this when planning your trip. On the day of Nyepi, the airport is closed (yes, closed), no taxis are allowed on the street and room blinds should be closed. Most hotels will allow you to freely use the hotel facilities, but you are asked not to leave the hotel grounds. Since a lot of the hotel workers are from Java, they are allowed to work on this day, while native Balinese get to practice their day of contemplation.
The next Nyepi probably falls on 25 March 2020. This is an estimated date, based on lunar cycles. It may be off by a day or two - or a month.