We have had many of those days. : ) Let me share a tidbit that worked in our family. We made them go into a walk-in closet or somewhere that is closed off and neutral territory for all kids involved (ie, not one of thier rooms!) and told them they couldn't come out until they solved the problem on their own. Life continued while they were in the closet. We have continued watching our movie, or continued with snack, we have even started and finished dinner while some of them were arguing it out. Once they come out, they realize all that they missed out on and hopefully get the message that life does not come to a standstill if they want to fight. In the meantime, I didn't got back and give them what they missed. No rewinding movie, no missed snack, or dinner. Nothing! They next time they started to fight, I would ask them if they need to solve it in the closet. Now they go in there less frequently, and come out much quicker in a better mood because they resolved it on their own.
I only get involved if their is physical aggression which at that point I seperate them and when they cool down, they try again. No freedoms until they have resolved the issue themselves. If I do have to get involved, they pay me for my time by doing a chore of my choosing together. Again, no freedoms until they are done. The kids get along so much better now and think before they decide if it is worth it to argue over such menial things. The cost of missing things is usually not worth it, so they drop the issue before it becomes an argument.
Expect the first time to last a while because they don't realize what they are missing yet. Each time after should be less and less as they see you follow through with not allowing them to makeup that missed time.
Let me know if that helps! : )