Kartik poornima is the birthday of Matsya, Vishnu's fish-incarnation. It is also the birthday of Vrinda, the personification of the Tulsi-plant:
According to Hindu mythology, the Tulsi plant was a woman named Vrinda, a synonym of Tulsi. She was married to the demon-king Jalandhar. Due to her piety and devotion to Vishnu, her husband became invincible. Even god Shiva, the destroyer in the Hindu Trinity could not defeat Jalandhar. So Shiva requested Vishnu - the preserver in the Trinity - to find a solution. Vishnu disguised himself as Jalandhar and violated Vrinda. Her chastity destroyed, Jalandhar was killed by Shiva. Vrinda cursed Vishnu to become black in colour and he would be separated from his wife. Thus, he was transformed into the black Shaligram stone and in his Rama avatar, his wifeSita was kidnapped by a demon-king and thus separated from him. Vrinda then buried burnt herself on her husband's funeral pyre or immolated herself due to the shame. The gods or Vishnu transferred her soul to a plant, henceforth which was called as Tulsi. As per a blessing by Vishnu to marry Vrinda in her next birth, Vishnu in form of Shaligram - married Tulsi.