There is but one decree. — He refers to the decree mentioned Daniel 2:5, that both the dream and the interpretation must be told. These two things must go together, for they form the subject of one decree.
Ye have prepared... be changed — i.e., “you have made au agreement among yourselves to postpone the matter till a more lucky time for explaining the dream shall come.” On Eastern notions about fortunate days, comp. Esther 3:7 and the standard inscription or Nebuchadnezzar towards the end.