What is a Gas Day ?

and how it contributes to the Hum getting louder at night.

The Document to the right describes a Gas Day and gives some explanation to the logic behind it.  As you can see there is no consideration given for when people are sleeping and the definition of a gas day is intended to serve the best interests of delivering gas when it is needed. 
The Gas Day explains why the Hum quite often conforms to a repeating pattern and in some it is predictable.