Choir precedence, that is, whether the left choir or the right choir is the first choir, is determined as follows.
The right and left choirs alternate daily in taking the turn of the first choir and second choir. In this case, a day is calculated as beginning with Vespers and ending with the Ninth Hour.
The cycle begins on Palm Sunday:
Palm Sunday: the left choir is first choir
Holy Monday: the right choir is first choir
Holy Tuesday: left
Holy Wednesday: right
Holy Thursday: left
Holy Friday: right
Holy Saturday: left
Pascha: right
Bright Monday: left
Bright Tuesday: right
Bright Wednesday: left
Bright Thursday: right
Bright Friday: left
Bright Saturday: right
Thomas Sunday: right (purposeful anomaly)
Monday after Thomas Sunday: left
And the cycle continues.
In order to prevent the choirs from always being first for Sundays of certain modes (for example, the right choir always being first for a Sunday of fourth mode), the choir that is first for the Saturday of plagal fourth mode is also first for the next day (the Sunday of first mode). The cycle then continues as normal.
Example:
Plagal fourth mode Friday: left
Plagal fourth mode Saturday: right
First mode Sunday: right
First mode Monday: left
First mode Tuesday: right