NN,
At the risk of sounding picky, I thought it was two WR days around a 7 day leave block. The NA day before a DFW is correct in that you can't be allocated a trip although you can be returned from a trip on that day. That gives you your total of 56 days per year.
As has been mentioned though, the Wrap Days and the Non-Assignable day attract no credit, so can effect the way you bid.