Anybody might ask, "What keeps the LEDs from turning on during the day?"
The answer is that the current provided by the Solar CD seeks out any open path. Most of the current flows directly from the Solar CD and into the batteries. But a small amount of current flows through 2.2k resistor R1, where it meets the higher resistance of 10k resistor R2. An even smaller amount of current flows through R2; the rest of this current flows into the "base" lead of transistor Q1, which "turns on" Q1, creating an open path for current to flow through 2.2k resistor R3 and then through Q1 from collector to emitter. This prevents current from flowing into the base of transistor Q2. If current flowed into the base of Q2, the LEDs would turn on.