Hello Richard. Your quotation from Turretin expresses the main point, namely, that the church is freed from any Mosaic obligation to worship on feast days, but is freed to worship. So, the first reason is Christian liberty. Second, the example of the apostles in Acts is that they worshipped on more than just the Lord's Day. The history of the early church's daily services (think, Origen, Augustine, Chrysostom preaching daily) stems from this apostolic practice.
I will have an article in the January 2009 issue of
American Theological Inquiry, which explains Lutheran adiaphora, but also scratches the surface of how that doctrine was appropriated by the Reformed in the area of feast days.
Blessings.