What is "dwadashi"

Dwadashi (also Dvadasi or Dwadasi) ( Sanskrit for Twelve) is the twelfth lunar day ( Tithi) of the shukla (bright) or krishna (dark) fortnight of every lunar month in the Hindu calendar.

Dwadashi is especially suitable for the worship of the sacred Tulasi tree (Tulasi stava of srishti khanda of Padma Purana), much like Ekadashi is especially suitable for the worship of Vishnu. The saint poet Annamacharya died on Phalguna Bahula Dwadasi (12th day after full moon) in the year Dhundhubhi.