Yar ngo sngon 'gro

A lunar month can be defined in two different ways, either running from new Moon to new Moon or from full Moon to full Moon. The first of these is called waxing-first (yar ngo sngon 'gro), meaning that the waxing fortnight comes before the waning fortnight. In this system, the waxing fortnight commences the day following new Moon, and culminates with the day of full Moon. The next day starts the waning fortnight which concludes on the next new Moon day. This is how the months are structured in normal Tibetan calendars, following the system defined in the Kālacakra Tantra and Vimalaprabhā.