It's connected with approaching weather fronts, a cold front will have a short leading edge with a short span of rain before clearing into clear skies (I'm not including occluded fronts here) and a warm front will have a long leading edge of rain/drizzle which will last for hours, as well as a longer time to clear through.

The theory is that as the sun is setting, if a front is approaching it will reflect off that leading weather front edge, the same in the morning sunrise. It's kind of an old way of trying to predict the weather for Shepard's and their flocks. Because if the cloud on front is short or long they could tell which kind of front is coming, or none of course. And if it's likely to rain for a long time or not.

It's also dependant which hemisphere you are in, in the UK for instance if you turn your back to the wind, the low pressure system (which is associated with crap weather) will always be on the left.