I have found that if you are willing to spend the better part of a whole day travelling in each direction, you can get FF awards just about any day of the week (on UA and AA at least). The biggest problem I have had is trying to book that elusive hawaii trip in first class on UA where I cannot book just the outgoing (330 days in advance) and book the return flight when it opens up 10 days later. This is due to my lack of status.

Other than that, if you are willing to fly out at 6:00 am and sit in a connecting airport from 10:30 am 'til 6:30pm then arrive at your destination around midnight, you can most likely find an open flight 6-9 months in advance of where you want to go.