Reh is one of the most important festivals of the Idus. The Idus believe that they are the sons and daughters of the divine mother Nanyi Inyitaya. But none can get her blessings and keep alive bond of brotherhood and social feeling strong, unless one performs the puja or celebrate the Reh festival. But it is so expensive that only a few people can afford to celebrate the festival for propitiation of the supreme creator, the great mother Nanyi Inyitaya

The festival is celebrated during February August. The people who inhabit snow fall areas viz. Talo, Amru, and Dri villages of the Dibang valley celebrate it during summer and monsoon i.e. during June August, when the climate is moderate and shoal of fishes are available in streams and rivers. The Idus in the other parts celebrate the same during February to May.

The festival requires a number of sacrificial buffaloes for offering to the great mother Nanyi Inyitaya. Presents such as money in cash and pigs are given to the relatives. The festival being very expensive, all arrangements and preparations for the festival have to be made four or five years before the actual celebration of the festival. As such a person wanting to celebrate this festival has to take resort to the system locally called Ada which is nothing but collection of mithuns, pigs, cash, money etc., even by way of loan from others. When Ada is completed atentative year is fixed about one year ahead of the actual celebration. The preparation of rice beer in large scale locally called Yunyiphri is under taken, three to four months before the actual celebration.