The tulsi plant was a woman named Vrinda - wife of demon-king Jalandhar.
Her piety and devotion made her husband invincible.
So, Vishnu took the form of Jalandhar and violated Vrinda.
Her chastity destroyed, Shiva killed Jalandhara.
Vrinda cursed Vishnu to become black; and also that he be separated from his wife, Lakshmi.
Tulsi vivah marks the north-east monsoon.
The household ceremony is performed any time between Prabodini ekadashi and Karthik Poornima.
-Wikipedia
