The Barah Maha is a Punjabi form: a poem of twelve stanzas, one for each month, in which a woman separated from her beloved describes the year through her longing.
Guru Nanak Dev Ji composed one in raga Tukhari and Guru Arjan Dev Ji one in raga Majh. Both take the folk form and turn the separation into the soul's separation from the divine.
The Majh Barah Maha is recited in gurdwaras on the sangrand, the first day of each month, one stanza at a time, which means the composition is read through once a year in step with the calendar.
The Nanakshahi calendar of 2003 took its month names from this poem, which is how a sixteenth-century devotional composition came to name the months of a modern reform calendar.
The form is used by every major Punjabi poet since, in both scripts, and it is one of the clearest cases of a folk form being taken into scripture without being flattened.
The form runs through the twelve months of the calendar, taking the weather and the work of each and using it for the state of a person waiting.
There are two in the Guru Granth Sahib Ji, by Guru Nanak Sahib Ji in Tukhari and by Guru Arjan Dev Ji in Majh, and the second is the one usually recited.
It is a folk form taken over. The barah maha was sung by women across the region long before it was written, and the versions in the volume assume the listener already knows the shape.