EWTN Catholic Q&A
Nine days
Question from Cindy on 04-08-2005:

What is the reason behind a novena usually lasting nine days? Thank yo and God bless.

Answer by Colin B. Donovan, STL on 04-12-2005:

The Apostles and the Blessed Mother spent nine days in prayer between the Ascension and Pentecost. This was the proto-novena of the Church. We pray for nine days in imitation of their example.