They open in a block. Just keep checking. The first few days, you will not see any specials - you will just see the regular fare. You need to keep checking.
Here's what I found with Airtran - I "did some trending" over the course of the last three weeks for my trip the 28th of November through December 5th:
Mid-day on Friday through Mid-morning on Monday seem to show the higher fares. Tuesday through Thursday seems to post the lower fares; they go down early in the morning, and then creep up during the day.
I booked mine at 6:30 am on a Thursday and by the time I called my friends and they got online, it went up about $40.00.