In the relay’s point of view, dark activated means the relay will be activated when the light intensity fall below a certain threshold. Without hysteresis, the relay will be activated and deactivated if the sensed brightness fall under or rise above a single point of darkness level. With hysteresis, the darkness level for activating and deactivating the relay will be different, and this solve the relay oscillation problem when the light intensity is swinging up and down around a single point of no hysteresis activation level.
We can employ the hysteresis of a 555 IC to improve the sensing of a drop in light, since the internal 555 circuit has 1/3 and 2/3 supply voltage thresholds. We have to use a LDR or CDS cell with aout 2 to 8 k resistance at desired light level. At the dark, the resistance of the LDR will rise and activate the relay if the voltage at pin 2 reach 2/3 of supply voltage (8V). After the relay is activated, more light is needed to make the LDR decrease its resistance until the voltage at pin 2 falls below 1/3 supply voltage (4V).