Originally Posted by DVTimes

Gary (pretending to be Nat) was trying to make a joke.

He does not understand that advent calendars may not be used in all countries.

To explain, in the UK we have advent calendars running up to christmas. These are box like things that you open each day. Inside is often a sweet or something.

What Gary (pretending to be Nat) was trying to suggest in his 'joke' was that someone has opened the windows of his advent calendar and stolen the sweets.