Some one correct me if I am wrong but I believe that it is based on the nine days the apostles spent in prayer from the time that Jesus ascended into heaven to the time that the Holy Spirit descended upon them. They prayered for nine days and on the nineth day their prayers were answered which is why most novenas are based on some varriation of ‘nine’ – nine hours, nine days, nine times, eighteen (two times nine), etc.
I found it interesting that there was a novena of preparation in the middle ages that was prayed before Christmas that had its origins in the nine months Our Lord was in His Blessed Mother’s womb from the Incarnation to the Nativity.
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