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?