Re: Mayoon-Mehndi??
Mayoon and Mehndi often look very similar.
As others have said, the Mayoon sort of officially announcing the beginning of the bride's preparation for the wedding. The tradition was that from that day on, she would stay at home and wear yellow and be pampered morning and night, in order to be ready for her groom.
The Mayoon itself was a time for the close family and friends of the BRIDE only (or if you're doing a mayoon for the groom, then for the groom only) to gather, sing, dance, do last minute preparations for the wedding, feed the bride mithai, etc. It's more casual then a mehndi because you don't have the other side present. Also, since some brides don't reveal their faces to the groom's side until the wedding day, the bride is more present and more involved in the mayoon than in the mehndi.
The Mehndi function was traditionally thrown by one side to host the other side. Since the tradition was for the bride and groom not to see each other, it often worked like the groom's side came to the bride's house to bring gifts, bari, mehndi etc, and to feed her mithai and give salami, etc. But the groom himself would not be present. The groom's side would host a separate mehndi where the bride's side would attend but she would not be present. Obviously, these days it often does not work that way. The mehndi is a joint function with both sides and both bride and groom present. The Mehndi is the official competition of singing and dancing between the two sides. It is also when the bride's sis (or female cousin/friend) wraps the groom's finger in mehndi and does not let go until he shells out some cash. Lots of fun and games.