What is the least frequent scheduled flight in the world?

We already have questions about the cheapest, shortest, most layovers and longest flights in the world. Now I am interested to know which scheduled flight has the least frequency.

The constraints are:

The flight has to be on a fixed schedule - so it's not a charter flight or a private flight
It has to have a flight number
Both airports must have an IATA code or an ICAO code
A regular person can buy a ticket - so it's not a military or government flight
The question is about flights, not planes - so it doesn't matter which plane is used on each leg

I know plenty of flights that are scheduled once per week, but perhaps there's something even less frequent?

Best Answer

I don't know if it's the least frequent, but LATAM offers flights once a month between Santiago and Mount Pleasant (on the Falklands) via Punta Arenas and Rio Gallegos. The outbound flight (to the Falklands) is flight LA897 operating on the second Saturday of the month, and the return flight is flight LA896 operating on the third Saturday of the month.

On the other Saturdays, LA895 (outbound) and LA894 (return) operate the same route, except without the stop in Rio Gallegos. So the RGL-MPN leg in each direction is only run once a month.

The reason is that flights are allowed under an agreement between the British and Argentine governments in 1999. The British did not want any direct flights from the Falklands to Argentina, only to Chile; the Chileans and Argentines did not want a flight without a stop in Argentina. This was the compromise, see http://www.nytimes.com/1999/07/15/world/britain-and-argentina-to-allow-commercial-falkland-flights.html.