One of the last two undeciphered Dead Sea Scrolls has been read by researchers at the University of Haifa in Israel.

The 900 scrolls, discovered in desert caves at Qumran in the 1940s and 50s, are the work of scribes belonging to an ancient Jewish sect and include some of the earliest biblical manuscripts.

The newly decoded document is made up of more than 60 tiny sections written in a secret code. They were originally thought to have belonged to different scrolls, but researchers Dr Eshbal Ratson and Prof Jonathan Ben-Dov showed they were from a single document.

The scroll – originally written in code – gives an insight into the unique 364-day calendar used by the sect. According to the researchers, this calendar was involved in one of the fiercest debates between different groups during the late Second Temple period. The lunar calendar, which Judaism follows to this day, requires a large number of human decisions. People must look at the stars and moon and report on their observations, and someone must be empowered to decide on the new month and the application of leap years. By contrast, the 364-day calendar was perfect. Because this number can be divided into four and seven, special occasions always fall on the same day. This avoids the need to decide, for example, what happens when a particular occasion falls on the Sabbath, as often happens in the lunar calendar.

'The Qumran calendar is unchanging, and it appears to have embodied the beliefs of the members of this community regarding perfection and holiness,' the researchers explain.

Dr Ratson told Haaretz that arguments about the calendar 'may be one of the reasons this sect left the Temple and went to the desert. They had many disputes and this was one of them – they couldn't celebrate holidays together.'

The scroll gives the most important dates in the Qumran sect's calendar, including two special occasions not mentioned in the Bible. It also provides, for the first time, the name of the day marking the transition between the seasons – 'tekufah' or 'period'.

According to the university, the scroll also reveals that the person who wrote the scroll – probably one of the leaders of the sect familiar with the secret code – forgot to mention several special days marked by the community. Another scribe corrected the errors, adding the missing dates in the margins between the columns of text.

'The scroll is written in code, but its actual content is simple and well-known, and there was no reason to conceal it,' the authors say. 'This practice is also found in many places outside the Land of Israel, where leaders write in secret code even when discussing universally-known matters, as a reflection of their status. The custom was intended to show that the author was familiar with the code, while others were not. However, this present scroll shows that the author made a number of mistakes.'