Ahh, you wish the LED's to stay on and not fade out?...
Good question, some kind of Latch then 
- probably require an IC of some kind to remember the state, someone else can probably explain how that works, all i can think of is a Latching IC, not sure how'd you do it with basic circuit components.
because as soon as the light level drops, the cap will slowly discharge making your LED's dimmer, you'd need a latch to stay delivering current until some kind of user input?