Henna and Its Origins
Mehndi is typically used in Indian or Middle Eastern weddings. The designs promise health, prosperity and fertility. Additionally, the groom's name or initials are often hidden in the design. If he can't find them, he owes the bride a gift.
Since the bride in those cultures goes to live with her in-laws, another side-effect of mehndi is that she cannot do housework until the artwork fades, giving her time to assimilate with her husband's family and get to spend time with her husband.