Bismihi Ta'ala

Hazrath Ayesha (Radhiallaahu Anha) narrates that Hinda, the daughter of
Utbah said, Oh Prophet of Allah, grant me Bai'at (allegiance). Rasulullah
(Sallallaahu Alayhi Wasallam) replied, I will not grant you Bai at until you
change your palms (apply mehndi) - (Mishkat pg.383 -Rahimiya)

Hazrat Ayesha (Radhiallaahu Anha) states that a woman behind a curtain
presented a letter to Rasulullah (Rasulullah (Sallallaahu Alayhi Wasallam).
Rasulullah (Sallallaahu Alayhi Wasallam) withdrew his hand saying I do not
know if this is a males hand or females hand, the woman replied, Rasulullah
((Sallallaahu Alayhi Wasallam) said, 'If you were a female you would have
changed your nails, (by applying mehndi)' - (Ibid)

In view of the above quoted hadith one of the reasons for females applying
mehndi is to distinguish her hands from that of a male. That can be best
achieved by applying mehndi on the palms and the nails. That can also be
achieved by applying mehndi on only the palms or only on the nails.

It is recommended for women to apply Henna but to establish functions like the customary Henna party before weddings is a hindu tradition and non-Islamic in which many Haraams such as music and free mixing of the two genders takes place. This is not permissible.

Also, a woman shouldn't go about displaying herself once she has applied Henna.
and Allah Ta'ala Knows Best