It does not say anywhere in the Bible to pray to Mary or anyone else But God.. It is a false doctrine and very un- Christian.. I am glad i left the catholic religon in my 20's and became a Christian..
To be perfectly fair here, I'm pretty sure that all the examples in the Bible of prayer are to God the Father. Which means that praying to dear Lord Jesus ( as many sweet little old lady baptist sunday school teachers did) or to the Holy Spirit (as many charismatic believers advocate and practice) are also unbiblical. I'll grant that since Jesus and the Holy Spirit are also part of the godhead the "error" of praying to them is a bit different than praying to Mary, but when it comes to the practice of individual people, it may be good to check to see just how biblical our behavior is before condemning others for unbiblical behavior.