Photo Cell timers are one of the easiest and most reliable timers to use with your Christmas Lights.  They have a photo eye on them and when it gets dark they turn the power on.  Most have a dial so that you can choose to run the lights for a predetermined amount of time.  An issue that can arise is when there is other light sources in the area.  Sometimes security lights, the Christmas lights them self eves or any other light source will make the timer think it is daylight again and turn the lights off.  The fix to this is to move the timer or use a digital one.