October is the month of the Holy Rosary.  Until about the 15th century hundreds of mysteries were part of the Rosary devotion, then the 15 mysteries that we know today were definitively fixed as "the Mysteries of the Rosary."  Pope Saint John Paul II, in his encyclical, Rosarium Virginis Mariae, in 2002 added the five Luminous Mysteries.
Through the meditations of the complete Rosary one recalls and has impressed on his mind, the Popes tell us, "the chief mysteries of the Christian religion," "the mysteries of our Redemption," "the great mysteries of Jesus and His Mother united in joys, sorrows, and triumphs." The twenty mysteries are divided into four equal groups, known as "The Joyful," "The Sorrowful," "The Glorious," and "The Luminous Mysteries."

Pope Francis invites the faithful to pray the Rosary in October

The Holy Father invites all the faithful, of all the world, to pray the Holy Rosary every day, during the entire Marian month of October, and thus to join in communion and in penitence, as the people of God, in asking the Holy Mother of God and Saint Michael the Archangel to protect the Church from the devil, who always seeks to separate us from God and from each other.

The Holy Father's Intentions for the Month of October 2018

Evangelization – The Mission of Religious:
That consecrated religious men and women may bestir themselves, and be present among the poor, the marginalized, and those who have no voice.

The feasts on the General Roman Calendar celebrated during the month of October are:

October 1 - Thérèse of the Child Jesus, Memorial
October 2 - Guardian Angels, Memorial
October 4 - Francis of Assisi, Memorial
October 5 - Bl. Francis Xavier Seelos; Faustina Kowalska, virgin, Optional Memorial
October 6 - Bruno; Bl. Marie Rose Durocher (USA), Optional Memorial
October 7 - Twenty-Seventh Sunday in Ordinary Time, Sunday
October 11 - St. John XXIII, Optional Memorial
October 14 - Twenty-Eighth Sunday in Ordinary Time, Sunday
October 15 - Teresa of Jesus, Memorial
October 16 - Hedwig; Margaret Mary Alacoque, Optional Memorial
October 17 - Ignatius of Antioch, Memorial
October 18 - Luke, Feast
October 19 - Isaac Jogues, John de Brébeuf and companions (USA), Memorial
October 20 - Paul of the Cross, Optional Memorial
October 21 - Twenty-Ninth Sunday in Ordinary Time, Sunday
October 22 - John Paul II, Optional Memorial
October 23 - John of Capistrano, Optional Memorial
October 24 - Anthony Claret, Optional Memorial
October 28 - Thirtieth Sunday in Ordinary Time, Sunday