NEC3 talks in 'weeks' for response times with no reference to working weeks, working days etc. . Consequently, a week means from Monday to Monday, Tuesday to Tuesday etc, regardless of whether that week includes non-working days such as weekends (which it always will) and bank holidays.
Having said that on a practical basis I would within a programme set up activities on a working calendar - including acceptance periods etc. If we are having to play "games" with response times (not that I am saying you are necessarily here) then the contract is probably not going very well.