The smallest regular backpacks I have are around 18 or 19L which is a nice size for a day pack. I don't do festivals, just hikes and camping trips. On a day hike I still like to have the minimum stuff I'd need for an overnight stay; to me that's pretty much the definition of a PSK.
“I'd rather have questions that cannot be answered than answers that can't be questioned.” —Richard Feynman