I know how to saturate in post-processing. But there are times of the day and the year when natural outdoor light is extremely saturated. Towards sunset in spring in southern England, for example. What is the physical quality that makes this light "saturated"? Is it a change in colour temperature? Or is there something else which affects perceived saturation?
I think chills42 is correct, a few factors compound to create vivid colors when the sun is low on the horizon. This affects the actual scene, i.e. the actual colors we see.
But there is another dimension to the situation: what the photos we take look like. The dynamic range of the scene during the golden hours is usually lower, allowing the camera to capture colors more easily. This happens to me all the time; a scene in bright noon sunlight looks completely vivid and saturated (imagine a field of flowers on a sunny day with a blue sky), but there is no way to capture it on a camera with a single exposure. I can underexpose the sky to capture the vivid blues, but then the foreground will be underexposed. Or I can expose the foreground correctly and the sky will overexpose and look white. I think this tricks some people (when they are viewing their photos) into thinking the scene had less color than it did.
This is usually due to a color temperature shift that occurs at certain points in the day, as the angle at which the light from the sun changes with the rising and setting sun.
Typically, lighting is softer (more diffuse) and warmer in hue, and shadows are longer. When the Sun is near the horizon, sunlight travels through more of the atmosphere, reducing the intensity of the direct light, so that more of the illumination comes from indirect light from the sky (Thomas 1973, 9–13), reducing the lighting ratio. More blue light is scattered, so that light from the Sun appears more reddish. In addition, the Sun's small angle with the horizon produces longer shadows.
These times are often referred to as the "golden hours", and there is an online calculator that can tell you the times that the light will be best in your area.