Can you clarify the effect that you are looking for?
Is your goal simply to turn on the LEDs after a certain delay period, leave them on for a specific time, and then turn them off for a specific period (and repeat), or,
is it to ramp up the intensity of the LEDs over a period of time to simulate sunrise, eventually reaching full intensity, leave it at that intensity for a period of time, and then ramp down the intensity to simulate sunset,
or something else?
Also, are you looking to vary the LED intensity by pulse-modulating the current that drives the LEDs and varying the pulse duty cycle or frequency, or some other method?
And, are you looking to use an array of ordinary LEDs that are typically used as indicator lamps, or do you plan to use high-intensity white LEDs that are used for flashlights?