There are some very complex explanations for The Diurnal Cycle. Most people when asked will say it tells you the 2 hours that a meridian is most active. While this is correct it has little practical use in Kyusho and striking. You would need to know what time it is...but what if the person had flown in from a different time zone? Would this affect your attack?
The most useful explanation for The Diurnal Cycle is that it explains the flow of Qi Energy through the body and the relationships between the meridians in respect to this Qi flow. Basically one can think of it as a schematic of the bodies plumping in respect to Qi flow.
All you really need to understand is the directional flow of Qi energy through the meridians and how to strike the meridian to either back the energy up or block its flow
The diurnal cycle flows in the order of lung, large intestine, stomach, spleen, heart, small intestine, bladder, kidney, pericardium, triple warmer, gall bladder, liver and back to lung.