On 24/7 holidays, weekends, special occasions, all ON.
Here's my reasoning and I'm sure others know this reasoning for leaving your PC on all the time.
What happens when you turn on your PC, current is shot through all of your parts turning everything on right? Everything heats up quickly. All your fans kick in, etc. Everything is all good to go. Then you get nice airflow going through your system and all the temperatures balance out.
What happens when you turn the system off every night before you go to bed? All the parts cool down. When you use the machine again (next morning or next day), you turn it back on and everything heats up again.
You can see where I'm going with this. What is happening is you keep on hitting that power button over and over, every day and you are doing more harm than good if you were to just leave your system on 24/7.
By leaving your PC on 24/7, everything stays around the same temperature and the airflow going through your case stays the same. It REALLY does not cost you anything at all (electricity bill wise) by leaving your PC on all the time. TRUST ME you don't notice it. Maybe $1 a day? MAYBE. It's worth it IMO.
Anyway..just my $.02
Joe