Originally Posted by xliioper

Bottomline, the cheapest fares almost all have a 21-day advance purchase requirement. If you are waiting to book less than 3 weeks out before your trip, there are few cases where you will find the best possible fare because the cheapest fares won't be available for purchase even if there is bucket inventory available. Beyond that, there are going to be bucket differences due to demand profiles. The most popular and convenient flight times on the most popular travel days (Sun/Mon departures, Thu/Fri returns) will often not have inventory in the lower fare buckets on flights. Having flexibility if traveling on those days (which you seem to have) helps. Summer travel also changes the dynamic as you are now competing with hoards of leisure flyers for seats. The slower travel periods (like May) will often have better fare bucket availability than higher demand months and periods around holidays.
Having familiarity with the published fares on a route can be helpful. If you see flights with the lowest published fares, then you can reasonably you are getting a good deal. It's possible they will revise the published fares at a later point, but generally speaking the changes aren't all that dramatic for the lowest fare classes.

For example, you can find that AA/UA/AS/DL all currently have the exact same $118.40 one-way fare on SEA-NYC (connecting flights are somewhat more because of additional taxes, but the base fare is still the same). If you look in the fare rules, you will these are seasonal fares as they are only good for travel through June 9th. They also have a 21-day advance purchase requirement and must be purchased by April 5th. The main difference is that these are Basic Economy fares on UA/AS/DL, while it's a main cabin fare on AA. The corresponding main cabin fare on UA/DL is $158.40 (+40) while AS is $153.40 (+35). The final gotcha is that these fares are only good for Tue/Wed/Sat travel. There are similar fares valid for Sun/Mon/Thu/Fri travel, but they are $18 more at $136.40.

Originally Posted by Often1

It is unlikely that you will find great deals more than 60 days out, so hold off on later tickets unless you do find a compelling price. Also worth remembering that with UA crediting (not refunding) the fare difference as well as accounting for trips you may need to cancel, those credits will build up and you will want to use them.

As to AC, it really does not help you because it cannot sell you a US-to-US connecting in Canada. There may be ways to avoid US law, but that becomes dicey, time-consuming and risky.