A novena is a series of prayers that are said for nine straight days, usually as a prayer of petition but sometimes as a prayer of thanksgiving. (See The Types of Prayer for more on prayers of petition and thanksgiving.) The nine days recall the nine days that the Apostles and the Blessed Virgin Mary spent in prayer between Ascension Thursday and Pentecost Sunday.
While the word novena comes from the Latin novem, meaning "nine," the term has also come to be used generically to refer to any lengthy series of prayers. Thus, the Saint Andrew Christmas Novena is recited for more than nine days, between the Feast of St. Andrew (November 30) and Christmas.