The best time to purchase a flight is when the prices are lowest. When that is, no one knows. Airline prices the last few years have been completely unpredictable. I would look at google flights and skyscanner. I think flying from Seattle, you may also want to start stalking the Delta website. Enter flexible dates and look at the website every few days or so. It is just a big guess, on my part, but you are fairly early so you could watch ticket pricing for a while. You could also watch for airline sales, often folks will post sales they've observed on the forum.
I never purchase nonchangeable tickets. So even after I've purchased, I might periodically look for a price decrease. If you are lucky and prices go down, you can rebook and get a credit which has to be used within a year. (That's for Delta, I'm not sure what the other airlines do, but I would guess, similar)
When you go to buy the tickets look at the various flight options. The difference between main cabin and comfort plus (again, Delta) is not always the same, and can vary by flight.
For us, from Minneapolis, a good main cabin price to say, London, Amsterdam, Paris would be anything around or under $1000. Lately, I find pricing around $900 and I jump all over it.