R4 sets the Hysteresis value, so when the op amp goes positive output, R4 will be in parallel with the 15k resistor, which is approx 13k, so your Ldr will need to get to less than 13k resistance before the led goes off, which is going to be VERY BRIGHT SUNSHINE,
Your resistor values are too low, change the 15k for a 1Meg,...
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