If you go on very long dayhikes (sunrise to sunset, 10+ miles) you want to pack enough to carry you over an extra day just in case.

You will want to ALWAYS have the 10 essentials in some form. (If you don't know what those are, cruise to the main page, backpacking.net, and check the links in the left column.)

My overnight (1-4 nights) pack is a 40 liter, however, my gear list has evolved and has less bulk/weight than most. My day pack is a 35L when I am out with a bunch of newbies for a dayhike or have special circumstances (fishing gear, dicey weather forecast, special occasion requiring a birthday cake or some other extra). My usual dayhike pack for up to 8 miles (which I can hike in half a day or less) is about 500 cu. in. and always has the 10 essentials, my food and water for the hike, a windshirt at minimum, a fleece or down jacket maximum, and my point and shoot camera.

I would say everything depends on what (or who) you take on day hikes. If you have kids, you may want a bigger pack, and 35L would be plenty.
"In the beginner's mind there are many possibilities. In the expert's mind there are few." Shunryu Suzuki