The term Novena comes from the Latin word novem, or nine. It is an ancient tradition of Christianity that involves devotional prayer in private or public for nine days (sometimes nine weeks). Our first period of Novena happened during the nine days between the Ascension and Pentecost where the disciples gathered in prayer.
In early Christian times, the Roman Catholic church would perform Masses for nine days with devotional prayers when someone died and soon grew to also associate novena with other themes - for example the nine months Jesus spent in the womb, the ninth hour of the Crucifixion, and the nine days of prayer with the disciples.
Novenas were officially and formally approved by the Papacy by Pope Pius IX when he approved a large number of novenas. The first chapter of the General Principles of Sacrosanctum Concilium (Second Vatican Council) gives us some guidance of where novenas fall in our Christian piety:
Devotions should be so drawn up that they harmonize with the liturgical seasons, accord with the sacred liturgy, are in some fashion derived from it, and lead the people to it, since, in fact, the liturgy by its very nature far surpasses any of them.