How to avoid Relay buzzing noise? (LDR dependent)
You need what is called a Schmitt trigger. And yes, you definitely need to put a capacitor on your power supply. Try something on the order of 1000 uF to start. Then measure the supply voltage, and it had better be more than 24 volts, although it almost certainly will be.

Then you need to get a comparator IC - an LM311 will do fine. You also need to get a cheap DMM to read voltages and resistances with - eBay will get you a basic unit for less than $10. Now measure the resistance of your LDR at the sort of light level where you want your lights to come on. Let's call this 1 Megohm, just to have a number to work with.

This circuit







simulate this circuit &ndash; Schematic created using CircuitLab

should work pretty well.