Two observances shape how the Guru Granth Sahib is actually encountered, and both are readings of the entire volume rather than of any passage.
In one, the volume is read straight through without interruption, a relay of readers taking it in turn so that the recitation never stops, and the reading concludes at an appointed time, usually about forty-eight hours after it began. In the other, the volume is read through over a longer period, with the reading taken up and set down.
Both are held to mark an occasion rather than to instruct: a birth, a marriage, a death, a move into a new house, an anniversary, the founding of something. The reading is the thing done, and the completion is what is observed.
For a reference work, the interesting fact is what these observances imply about the volume's organisation. A book arranged for continuous recitation does not need a beginning that explains itself, and the Granth does not have one. It does not need a conclusion that gathers the argument, and the Granth does not have one of those either. What it needs is to be finite, fixed and complete in a known order, which is exactly what the rag arrangement and the invariable pagination deliver.
The atlas records the observances as facts about the tradition. It does not describe how they are performed, which is not something to be learned from a page of description, and is not this work's to teach.