Bad Training Days Ahead...
How do you know when you are going to have a great day in the weight room and when you are going to have a less than stellar day? Is there a way to predict this?
Not really. That's probably not the answer you want but it's not easy to predict everything that you will feel on any given day.
There's a way around this though, and it's terribly simple: Always cut the last set short a few reps – and for many, this might mean to just do the requisite reps of the day, even if it's easy. This seems to work across the board.
What this does is ensure that you make progress each day without having the nasty training hangovers that occur after being party to the "All out or die!" training day.
Now I'm not saying to never have those, but when you do, realize that you'll pay later in the week for them. You can't continue that sort of effort day in/day out without suffering side effects, namely lack luster training and feeling like shit. This is especially true when squatting and deadlifting, as these take the most out of you physically and mentally. Many people have found out that cutting these sets short allow you to train better for longer.
Don't get caught up in training for the day – sometimes it can be a great way to relieve stress and take your mind to different places, but look at the big picture.
As a side note, this is precisely how I handle my conditioning. Most of my conditioning is done without much intensity – I relax and just get a quality workout in and leave. Do this consistently enough, however, and your conditioning goes through the roof.
That said, I usually have 1-2 days every month where I really PUSH my conditioning – I do something kind of stupid and basically kill myself. I pay for it later, BUT I don't do this all the time; I simply strive for good workouts done consistently. You'll be surprised at what happens when this is done, and it doesn't take long to realize the benefits.