Humidity does change the speed of sound in air but it's not much of a difference. Same with ambient pressure. I suspect, however, that it's one of two (well, three) possible situations.
One, the power quality from your Utility won't be consistent from hour to hour, and depending on where you live, may be either pretty decent or severely compromised during high demand periods. If you like, you could buy a Kill-A-Watt meter and see if your voltage sags or the utility is under-capacity during high demand which will cause the frequency to fall below 60Hz. More of a potential problem where you have conventional versus switch-mode power supplies driving your components (likely the case for at least your power amp/receiver/integrated amp).
You are also tightly tied to your closest neighbour's power supply (everything after the transformer on the pole, which services more than one residence, or more than one unit in a multi-unit building or complex) and whatever they do can affect your power delivery or power quality.
Nasty possibilities are X-10 devices, any "data over power" devices, appliances with compressors (fridges, freezers) or motors (furnaces, washers, dryers), microwave ovens, especially if they are aged and working hard to operate, and any switch-mode lighting (CFLs, LEDs). They all will affect Power Quality on an intermittent basis.
Two, your components may not perform optimally until they reach stable operating temperature, which can take up to half an hour (rarely, an hour) depending on your specific equipment.
Speaker break-in is a one-time thing; once the materials have been exercised to achieve a "broken-in" state, they won't change much, if at all, for their lifetime. (And for the non-believers, the Theil-Small parameters will measurably change from when a new driver is first run up to the break-in point; so yes, it is easily measurable). But in your case very unlikely to be the problem.
The "third" possible situation is your state of mind, where the system may sound worse if you are not relaxed for whatever reason.
If you are willing to do a bit of work, you could download the Audio DiffMaker application (WindowsOS, free) and see if, and by how much, the system does reproduce music differently at various times. If there are Power Quality issues you should see something show up.