The mehndi at Kavi’s sari day party was a huge hit — many people attending had never had mehndi done before, and the artist we found worked very fast, so everyone who wanted some decorating got some, I think — Kavi even got to go back at the end and have a second hand done. The artist mostly freestyled the designs, but she was also happy to take a photo Kavi had brought and reproduce it for her.
“Mehndi is a popular form of body art in South Asia and resembles similar traditions of henna as body art found in North Africa, East Africa and the Middle East. There are many different names for mehndi across the languages of South Asia.”
Mehndi typically lasts 1-3 weeks — mine was gone after a week, but some of Kavi’s friends it’s barely faded. Skin chemistry, I guess? I will not accuse them of not washing their hands. 
I suspect mehndi is a great way to test out a tattoo, if you’re thinking of getting one!
“Mehndi is typically applied during Hindu weddings and festivals like Karva Chauth, Vat Purnima, Diwali, Bhai Dooj, Navratri, Durga Puja, and Teej. Muslims in South Asia also apply mehndi during Muslim weddings as well as festivals such as Eid al-Fitr and Eid al-Adha.”
Although it’s a traditional part of weddings, I’d say that mehndi has spread broadly enough that it’s not really a religious / sacred practice — unmarried women will often get mehndi done at festivals, for example.
So if you’re worried about cultural appropriation, I think you’re pretty safe here. If you’re at a South Asian party where someone is doing mehndi, don’t hesitate to have some done! (In general, if you’re invited to participate in the culture, you’re fine fully participating, I think — I think you should also feel free to wear a sari to a South Asian wedding, though if you’re concerned, you can always check with the family to confirm.)
More about mehndi here: https://en.wikipedia.org/wiki/Mehndi
Locals, if you’re interested in booking this artist, let me know! I have her contact info somewhere, and can dig it up. Very affordable.
You can also get mehndi temporary tattoos online at Amazon, Etsy, etc.
