bgammill, welcome to FlyerTalk. In general -- and it's only general -- airfares of the type you're looking at are least expensive 21 or 14 days or more from travel dates and get more expensive closer to departure. Often this happens at the 7-day and 3-day (prior to departure) point. There are many exceptions depending on competition in the specific pair of cities you're looking at.

Fare changes can occur daily or even more than once a day. It's not unusual for $20 or higher fluctuations to come and go. Sometimes lower priced seats will exist on some flights that day and not on others. Flights may be lower on Tuesday - Wednesday and Sat., than on other days. With airfares, it's largely unpredictable.