I once bought a book for a summer trip. Honestly, I was looking for some light reading I could enjoy while sunbathing, so I randomly picked up a book with a butterfly on the front cover. It was En el tiempo de las mariposas (In the Time of the Butterflies), by Julia Álvarez, a Dominican writer.

When I started reading it, I soon figured out this was no light beach read. It was, in fact, a story that changed history…especially for women. It was the story of the four Mirabal sisters, and how the day three of them were assassinated became the International Day for the Elimination of Violence against Women.

The Mirabal sisters, Minerva, Mate, Patria and Dedé, all lived in the Dominican Republic in the 1950s, during the Rafael Trujillo dictatorship. They were all young and married, and had grown up in a family of wealthy farmers.

Minerva became a lawyer and, following in her uncle’s footsteps, got involved in the political movement against Trujillo. She and Trujillo met once, in 1949 at the San Cristobal celebration. The story goes that the dictator tried to seduce Minerva, but she rebuffed his advances. Trujillo never forgot or forgave the snub.

Minerva’s sisters, Mate and Patria, soon followed her into political action against Trujillo. Dedé joined later, even as her husband, Jaimito, tried to hold her back from doing so. Finally, the four sisters formed a group called the Movement of the Fourteenth of June (named after the date of a massacre Patria witnessed that compelled her to get into politics). But within the group, the sisters called themselves Las Mariposas (the Butterflies) after Minerva´s underground name, Mariposa.

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The Butterflies were not an active terrorist group, but rather distributed informative pamphlets about Trujillo’s victims. As a result, Minerva and Mate and all the sisters’ husbands, except Jaimito, were incarcerated several times. Still, despite these imprisonments, they never gave up fighting to end Trujillo’s leadership. They simply refused to give up on their mission to restore democracy and civil liberties to their country.

On November 25, 1960, Minerva (34), Mate (24) and Patria (36) went on a trip to visit their imprisoned husbands. Driver Rafina de la Cruz was with them. On their way back home, Trujillo’s henchmen stopped them and all were clubbed to death. Trujillo’s men put the women’s bodies back in their Jeep and ran it off a mountain road to make it look like an accident.

But everyone knew Trujillo was the one who ordered the assassination of these women. They had become very well-known and admired all over the Dominican Republic, and represented a great threat to his regime. Dedé Mirabal was the only sister who survived to tell their story, as she didn’t accompany The Butterflies that day.

The murder meant the beginning of the end of Trujillo’s regime, as the Dominicans wouldn’t stand for this cruel act or the continued tyranny of the bloody regime. Trujillo thought he was finally rid of a serious threat to his rule, but history found a way to avenge those assassinations.Trujillo was assassinated six months later by one of his associates.

The Butterflies have become symbols of both popular and feminist resistance throughout Latin America. Every November 25—the anniversary of the death of The Butterflies—women’s rights activists have observed a day against violence.In 1999, November 25 was designated as the International Day for the Elimination of Violence against Women by the UN General Assembly, in commemoration of the sisters.