Bishop Andrew (Heaven and Earth in little space) argues that the concept of Eastertide as Fifty Days (a week of weeks) is not subverted by the Pentecost Octave.
There is great significance in the notion of the eighth day, as Alexander Schmeman reminds us. Up to the time of S Basil the Great, Sunday was the 'eighth day': after six days of creation and a day of rest, the resurrection inaugurates the new and eternal day. There were eight people in the Ark, the first letter of S Peter tells us, and Baptism too is salvation through water through the resurrection of Christ. The risen Christ appears after eight days and this makes the liturgical octave fundamental not only to Easter but other feasts too. What is Pentecost but the eighth day after seven times seven weeeks? What is the Pentecost octave but the eighth week after seven times seven weeks?
Lawrence Hemming's Worship as a Revelation began the radical rethink, the genuine post-post-conciliar ressourcement, of liturgical thinking. Anybody concerned with this whole business should put Burnham onto their bookshelves just to the right of Hemming.