Tulsi Vivah , a sacred occasion in Hindu religion falls on the day of Devuthani Ekadashi , one of the 24 Ekadashis celebrated in a year. On this day, the annual ritualistic marriage ceremony of Tulsi or holy basil plant and Lord Shaligram, a form of Lord Vishnu takes place. It is believed that on this day, Lord Vishnu awakes from his cosmic sleep to begin Hindu wedding season. This year, Tulsi Vivah is being celebrated on November 13 (Wednesday), while the Dwadashi Tithi will begin on November 12. Here are all the details.
People who celebrate the festival perform a special ritual on Dwadashi tithi, the 12th day of the lunar month that typically falls in October or November. Tulsi Vivah or the ceremonial wedding of a tulsi plant or holy basil and a shaligrama or an amla branch (symbolising Lord Vishnu) takes place on this day as per the shubh muhurat or Tithi, which signifies the end of the monsoon, and the beginning of the wedding season. Couples or those wanting to get married often partake in this ritual to get blessings for a prosperous married life.

For the Tulsi Vivah ritual, the holy basil plant is decorated like a bride complete with bright red sari, jewellery, and vermilion. Lord Shaligram or amla tree branch is decorated as the groom. The rituals include aarti, offering of flowers, fruits, and sweets.
If you are living in the USA, here are Tulsi Vivah timings for different states as per USA time zones:
(Picture courtesy: Freepik, Agencies)