The actual notion of 8 glasses a day originates from a 1945 US Food and Nutrition Board which recommended 2.5 litres of daily water intake. But what is generally forgotten from this recommendation is, firstly, that it was not based on any research and that secondly the recommendation stated that most of the water intake could come from food sources.
All food has some water in it, although obviously fresh juicy fruits will have more than, say, a box of raisins. Suffice it to say that by eating regular food and having coffee, juice or what have you, you will end up consuming 2 litres of water without having to go seek it out specifically. If you find yourself in a water deficit, your body has a very simple mechanism for letting you know. Put simply, you will get thirsty.
If you are thirsty, drink water. If you are not thirsty, then you do not need to go out and purposefully drink 6-8 glasses of water a day since you will probably get all the water in your regular diet. One important caveat to remember though is that on hot summer days, your water losses from sweating go up and if you plan to spend some time out doors, having water with you is important to avoid dehydration and heat stroke. While the thirst reflex is pretty reliable, it does tend to fade with age and older people are more likely to become dehydrated without realizing it. Thus, the take home message is drink water when you are thirsty, but on very hot days it might not be a bad idea to stay ahead of the curve and keep hydrated.