Is it easier to install when the air temperature and/or the temperature of the window is warmer or cooler? Does it matter if there is direct sunlight on the window during installation?

If the window is in a heated/cooled space, it really does not matter what time of year you install. If the glass gets really hot in the direct sun though, it will dry out very quickly and make for a poor install. If you can do the install when there is no direct sun on the glass, it will be an easier process.