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Putting the LDR between 555 timer pin 7 and the tied together pins 2 and 6, will make it so that the LDR is what primarily sets both the high and low output timing based on light level. That is, as long as the +5V to pin 7 resistor is of relatively low value.
Remember that pin 7 connects to ground internally while the output is low. Therefore you need to make sure that the current flowing through pin 7 isn’t too high. It needs to sink the current through the resistor from the +5V supply voltage, plus the current from the capacitor when there’s a lot of light on the LDR.
LDRs typically have very low resistance under bright light.
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