In general when looking for the cheapest flight to St. John's, try to book your flight during the month of May for the best deal.
In general, most flyers find the cheapest time to book a flight to St. John's is about 49 days prior.
From most locations, the cheapest flights to St. John's are found on a Tuesday. Booking a Saturday departure is usually the most expensive.
Typically, cheap prices to St. John's are most often found for afternoon flights. Cheap prices are less likely for evening flights.