Mehndi is the application of henna as a temporary form of skin decoration in India, as well as by expatriate communities from the country. The word mehndi is derived from the Sanskrit word mendhikā. The use of mehndi and turmeric is described in the earliest Vedic ritual books. Haldi(Staining oneself with turmeric paste) as well as mehndi are important Vedic customs as a symbolic representation of the Outer and the Inner Sun. Vedic customs are meant to awaken the "inner light and so the gold of the inner Sun has an important symbolic function.
(Source from Wikipedia)
Mehndi is a ceremonial art form which originated in ancient India. Intricate patterns of mehndi are typically applied to brides before wedding ceremonies. The bridegroom is also painted in some parts of India. In Rajasthan, the grooms are given designs that are often as elaborate as those for brides. In Assam, apart from marriage, it is broadly used by unmarried women during Rongali bihu (there are no restrictions to the married ones). Henna is also common in some gulf states, specially Yemen, where the night before the wedding night is dedicated to decorating the bride with henna, and called "Henna night"