It doesn't really matter, except it does! Let me explain...
Lets say an airline has a flight with 100 seats. The lowest fare is $150, the next is $175, and the next is $200, and it keeps going up until you pay full fare. The airline will have 10 seats at the $150, 20 at the $175, and 20 at the $200 and so on until all 100 seats on the plane have a price. Basicly, what this amounts to is the fuller a flight gets, the more expensive the ticket, because (duh) the cheaper seats book up first.
So, even though the lowest airfare ($150 in this example) is offered on all flights for everyday of the week, it may not be AVAILABLE on the date/flight YOU want to book, because all the $150 seats got sold to other people. If you book far enough in advance, it's not an issue because all the $150 seats are open. But if you wait until 1 month before, all the $150 may be gone by then on some or all of the flights.
What this means to you: Try to book your travel on the days that have the lowest number of people traveling. Usually for Orlando, that means avoiding Friday, Saturday, and Sunday. Monday through Thrusday should be the best. As for time of the day, pick the times that have (generaly) are the least popular. Usually the most popular are flights that leave between 8 and 11 am, and after 5 pm (a lot of people try to leave home right after work, or want a full day in the park before returning) so you'd want to avoid those times if you can.
And again, if you book far enough in advance, those cheap fares should still be available on most or all of the flights, so again, it won't matter.
Confused yet?
