Have you ever hated the sound of your system one day, and loved it the next?

+1 ieales

except that the “properly functioning system” although representative of the majority of “Hi-Fi” is NOT at all high fidelity if all these extraneous factors audibly affect the sound. It is possible to find electronics equipment that is not weather dependent and speakers that don’t change drastically due to voice coil temperatures - of course few folks here are interested in that kind of performance as it would spoil all the ludicrous fun with fuses and ordinary bits of wire (for which audible changes are generally chalked up to - instead of the real culprit which is the poorly designed component equipment which rarely sounds the same twice)

+1 ieales

It's even more annoying when each song on an album needs a different level. 

Uh, chaps, level is one of things you CAN control. It is not what this thread is about, which is why the system oft sounds different day to day for NO APPARENT REASON. It’s a mystery. Level has nothing to do with it. Weather affects the power grid, thus even high end designer electronics, which apparently you chaps are completely unfamiliar with, one must conclude at least, are subject to the vagaries of the weather.

Pop quiz: has anyone who’s paying attention noticed his system sounds really good for no apparent reason the past few days?

Notwithstanding my point that you should concentrate on the music, not the sound, there is no doubt that the quality of the sound can vary from day to day.  And that difference can be significant! 
Right now I’m listening to a CD (Shostakovich’s 8th Symphony with Gergiev conducting) which sounded dull and lifeless a few days ago. Today it’s blowing my socks off!
The trick is not to let that bother you so much that you can’t get past it and hear the music. This can be difficult with very dynamic music.