When was the LAST time (day, month, year?) when there was NOT a plane/jet anywhere in the sky anywhere in the world?
Obviously - lots of days like that before the Wright brothers --- and some number of days AFTER the inaugural flight... but when was the last day there were no planes in the sky?
9/11 isn't the answer - FAA grounded flights in north america - but New zealand was not affected by that incident.
My guess? either just before WWII or very soon after WWII
ps. this is a question similar to, "When was the very last time your Mom or Dad tucked you in at night?" no importance to the answer in any tangible sense - but interesting never-the-less.
There is no way to know for sure. You might be able to determine when the last day there were no scheduled flights was, but during the early part of the last century pilots were not filing flight plans with a centralized agency, so there are no records that exist that one could use to be absolutely sure.