I assume you are talking more about how best to power the LEDs and not how to control them. There are lots of switcher chips designed to power LEDs. These would be the most efficient. Many of them have control inputs that support PWM dimming. The datasheets will give you lots of details. for example, check out this one from linear **broken link removed** It handles a wide range of input voltage, can drive up to 4 LEDs and supports PWM control. I'm not pushing this specific chip, it's just an example of what you can find. National, Linear, Maxim and others have numerous choices.
edit: I just realized you wanted a lot of LEDs - look at this one **broken link removed**
On the actual control, this is pretty straight forward. you have a time of day clock, a calendar function (driven from the clock), a computation of SR and SS based on the calendar (and location), computation of the desired brightness and then use that to set the PWM level. Your brightness level would basically be a divider of the PWM level. all that stuff is discussed ad nauseum on the internet - google is your friend.