Day and night detectors are mechanisms in which allows for the player to detect what time of day it is. In 1.5, a daylight sensor can easily be used to achieve this. However, prior to 1.5 a BUD was used to trigger redstone contraptions.
Material of housing (Stone, wood, whatever your house is made of.)
1 daylight sensor
Making the circuit
Place the detector in an appropriate place, near your house.
Place your lamps in an outer wall (They must be visible from inside!)
Connect the detector to a NOT Gate (If you're confused, check the picture), for best effect have a distance of 5-6 blocks (Again, if you don't understand, check the picture).
Connect the NOT gate to the lamps.
Thanks to the 1.8 update, there is an easier way to do this:
Place the detector in an appropriate place, near your home or on top of your home.
Place your lamps in an outer wall (They must be visible from the inside!)
Connect the detector to the lamps.
Right click the detector
This circuit can be used for many things, including
Turn on lights inside/outside
Turn on in-ground "street lights" to prevent mobs from spawning in a certain area
Use lights to prevent spawning, and combine with other defenses to keep mobs out of your house (For example, you could make a drawbridge that retracts at night, or something to automatically kill mobs outside your door in the morning, hooked up to the same circuit that will reduce mob spawning in the first place).
Use an "AND" gate to create a circuit that will only work at day, or one that will only work at night
The lights, when off.
The lights, when on.
This method of time of day detecting uses a block update detector.