"The electors of President and Vice President shall be appointed, in each State, on the Tuesday next after the first Monday in November, in every fourth year succeeding every election of a President and Vice President."
As far as whether a Federal election would be rescheduled, the Congressional Research Service published a study on this question in 2004: "Postponement and rescheduling of elections to Federal office". The report notes that while Congress has the authority over the timing of presidential elections, there is no express constitutional direction or federal law which "provides express authority to 'postpone' an election." Follow the link for the report to read more about the issue!