The first fifteen tithis (Phases of the Moon) comprises the sukla paksha, literally “bright fortnight,” the period when the moon is waxing where the moon grows into a full moon. This period is called Valarpirai tithi-kkal. The fifteenth tithi (phase/ waxing phase) is the full-moon day, termed as Pournami or Purnima (sanskrit).
The names of the Valarpirai tithis are: Prathamai (Pratipada), Dvitiyai, Tritiyai, Chaturthi, Panchami, Shasthi, Saptami, Ashtami, Navami, Dasami, Ekadasi, Dvadasi, Trayodasi and Chaturdasi. These repeat, beginning
with Prathama at the beginning of each paksha. The 15th tithi is full-moon day, Pournami. The 30th tithi is new-moon day, Amavasya.