In the video above, I visually explain how to make lights that only turn on at night. But if you don't want to be nice and watch the video, I will explain it in writing here. First you need a daylight detector. Then you want to lay redstone to 8 blocks away. Here you want to place a block. On the opposite side the redstone, place a redstone torch. This is essentially an inverter. At this point you most likely want to wire it underground. Once it is underground, invert the signal again. At this point you can branch off to however many places you want. Now wire the redstone up so that it is going straight into a block. On top of that block, place a redstone torch. On the space above the torch, place a redstone lamp. There you go you are finished. Now let me explain WHY it works.

The reason redstone must be wired 8 blocks away is that 8 is the lowest light level mobs can't spawn at. You will want the lights to go on just before mobs spawn as, the whole goal of this is to prevent mobs from spawning. Now the reason the signal is inverted 3 times (2 and a vertical wire, but same thing) is that you want to hide the redstone. Another point to this contraption is to not just have redstone wired up to a lamp (as then you could see the redstone) So by doing it this way, redstone is almost entirely hidden (you will need a couple redstone to be exposed because you need to wire away from the daylight sensor). The lights also only turn on at night (actually sunset I believe) So that you don't have a bunch of annoying torches all over the ground, but mobs still can't spawn.

I hope you enjoyed.