My understanding is that technically, you can't have a scheduled flight less than once a week, because airline schedules are published in a weekly format:
MH172 KUL0850 – 1200DEL 738 245
MH190 KUL1850 – 2150DEL 738 134
The last number in each row is the days of the week when the flight operates, from 1 = Monday to 7 = Sunday.
The only way to schedule a flight less than once a week would be to start and stop it every week when it flies, which is (again, technically) indistinguishable from a seasonal or chartered flight.