It depends on the airline, if you use someone like Air Transat or Zoom, then the
flight price keeps going down IF they still have empty seats on the
flight because they want it to be fully booked, but Air Transat and Zoom rarely have empty seats at any vaguely popular time of the year. I'd say avoid public holidays like Easter and Canada Day. At popular times the prices keep going up because they know they will sell them all.
If you use a big carrier like
Air Canada or British Airways, they usually sell blocks of seats to travel agents, so if you use someone like Thompson's or some of the big travel websites you can get a lower fare than booking it direct on popular routes. What happens then is any unsold seats get kicked back to the airline a couple of weeks before the
flight goes and those tickets will be dirt cheap. After that you get hit with higher prices because there are only a few seats left and those are usually bought by business travellers who get short notice of the need to fly somewhere.
So I'd say aim to buy the ticket about three weeks prior (unless you know for certain it's likely to be a full
flight), unless your travel agent gives you an okay price.
Steve.