In the end of August, when melon and watermelon season is ending, it used to be very popular to make these lanterns out of watermelon and melon.
In Mallorca (Balearic Islands), it was a very widespread tradition until the 1960s. Adults emptied melons and watermelons and scratched the surface to make drawings and some holes and put a candle inside to turn it into a lantern for children to play with. The scratched parts of the skin are so thin that they are translucid. The lanterns were also used to decorate the balls that farmers did to celebrate the end of fig-picking season.
This lantern was a toy that children used to play pretending to be the watchman (the watchman/sereno was the man whose job was walking around at night holding a lantern and keys to apartment blocks, and served as a safety reference; for example: walking people home, stopping fights between drunk people, calling a midwife or a doctor when needed, opening the door for people who forgot their keys, chasing after thieves, etc. It was common until the 1970s). Children also sang folk songs about the watchmen while they played with the lanterns.
In the 1960s, when Mallorca moved from being an agriculture-based society to a tourism-based one, traditions like this were abandoned. The towns of Alcúdia and school families associations in other Mallorcan towns still hold an annual Festa de les Llanternes ("Lanterns Festival" in Catalan) where they make lanterns out of summer fruits and children parade the streets carrying their lanterns and singing the watchman folk song.
In many parts of the Valencian Country, children also liked emptying watermelon to make lanterns and sing songs about the watchman.
It still persists particularly in the Ribera del Xúquer and Horta de València areas, and it has also become part of the festivities in places like Albalat de la Ribera and Agres —where every year the town's children hold a parade with their lanterns, accompanied by traditional music—, and Benicarló and Carcaixent —where neighbourhood associations organize yearly workshops to make them.
Photos and information from Ajuntament d'Alcúdia, Vilaweb, Sarau Alcudienc on Ara, Coordinadora d'APIMA de Sa Pobla, El blog de Maria M., Albalat de la Ribera, Etnoblog by Museu Valencià d'Etnologia, and Vilaweb.