Pakshavardhini falls on the 12th day of the Shukla Paksha and Krishna Paksha (Waxing and Waning Phase of moon) in a month in a Hindu lunar calendar. To be termed as Pakshavardhini Dwadasi, the 12th day should precede two days before Purnima (full moon) or Amavasya (No moon).
If it is Purnima or Amavasi tithi after two sunrises from 12th day or dwadasi then such a dwadashi is Pakshavardhini Dwadasi.
The day is considered highly auspicious by the Vaishnava followers. Some people observe a fast on the day. Sri Hari Vishnu is propitiated on the day.