I go to NY once a month at similar times. That is the busiest time, so you have to book further ahead to get the lower fares. Once those $19 fares are sold out, they don’t come back because they are nonrefundable. I try to book at least 2 months ahead for the winter and 3 months ahead for summer to ensure I get the cheapest tickets. If there’s a holiday, you need to book even further ahead.