How soon do bookies make betting odds available and why at that time?

In a league where all games are known when the league starts, how soon before the game starts do bookies provide pre-match odds? I have noticed that odds for all games in a league, like the English Premiere League, are not provided right away until a number of days before the match is played. Why is done like this? Why not provide odds for all games at the onset? Why wait till a few days to the game? What is the logic behind all this?