As bamboo bars have limited length, ancient Muong people made careful consideration to select symbols to inscribe on them to create the Khao Roi (also called Doi) calendar. 

Basic symbols on Khao Roi calendar:

The Khao Roi (Doi) calendar of Muong people uses simple symbols. 

 These three simple symbols are used to demonstrate Mua (rainy), Ca, Lo and Roi (Doi) days, among others on bamboo bars. 

For example, the V represents the Ca day, the X represents the Khoang Kac days, and a vertical line with small holes atop represents the Doi day. 

Symbol of Khao Roi (Doi) day

Muong people named their bamboo calendar after the Roi (Doi) constellation. The Roi (Doi) day is the one when that constellation passes in front of the moon, once a month.

On bamboo bars, the Doi day is demonstrated with a vertical line with one round hole on top of it and two - three others arranged symmetrically on both sides. 

However, the calendars collected in Lac Son district do not contain the Roi (Doi) day symbol. 

1. Single-factor days

Single-factor days in the Roi calendar are those when only one factor occurs such as the Ca, Hao, Lo and Roi days, according to the Muong folk belief.

 Figure No 1: Symbols demonstrating single factors on the Roi calendar

The Khao Roi calendar of Muong people has eight single-factor days, namely Ngai Lo(Lo day), Ngai Haw(Hao day), Ngai Ka(Ca day), NgaiKhaw Roi (Doi)- Roi (Tua Rua) star day, Ngai Muo Paw(rainy day), Ngai Khwang Kac(Khoang Cac day), Ngai Roi baw(the day when Roi comes), and Ngai Roi tha(the day when Roi goes). 

2. Multi-factor days 

Multi-factor days are those believed to record the occurrence of many factors. In other words, they are the days affected by at least two factors such as Ca and Hao or Hao and Lo appearing on a same day. 

Muong people believe that multi-factor days are bad ones. 

A preliminary survey showed that there are more than 20 multi-factor days in the Khao Roi calendar./.

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