During his pontificate, Pope Leo XIII wrote twelve encyclicals on the Rosary.
The first such encyclical was Supremi apostolatus officio of September 1883.
Most were issued in September in anticipation of October, which Leo would have dedicated to Mary, in particular through the Rosary.
The first encyclicals established the rosary as a public devotion.
[1] The following 12 encyclicals of Leo XIII are about the Rosary:[1]