15 days is one Fortnight and is called a Paksha.
There are 2 types of Fortnight, Bright and Dark, Shukla Paksha and Krishna Paksha, Tad Shuklam Krishnascha.
Fifteen Days after Black Moon / New Moon – Bright Fortnight / Shukla Paksha
Fifteen Days after Full Moon – Dark Fortnight / Krishna Paksha