A Novena is a prayer or series of prayers prayed either alone or in a group for nine days with a very specific intention. There are some novenas that follow the liturgical year, such as the Seven Gifts of the Holy Spirit, which is prayed from Ascension leading up to Pentecost Sunday. Other novenas are directed specifically to the Sacred Heart of Jesus, the Blessed Mother, or a specific Saint due to the virtue they possess and the need you have for the specific intercession. Other Novenas are directed at circumstances such as praying the Novena for the Souls in Purgatory or the Divine Mercy Novena.
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Visit the EWTN website, as one example, for Novena details.