Diurnal! Yes, but not always.

[jitami]

I found Ella in the driveway when she scared the crud out of me by slithering past my feet in the dark. I want to say it was fairly late... I'll have to look at her "found" thread It was after a hot summer day. Just checked and it was around 10:30 pm in late August.

[infernalis]

This would all make perfect sense.

Garters like cooler temps, food will be most abundant at dusk and dawn.

Combined with the only times I have ever witnessed a wild garter out cruising during the day was in the wetlands, in the shade. (excluding overcast wet rainy days)

Every single specimen I have ever documented, captured or witnessed during the daylight was under something, or right after it rained out.

Cheeto caught me totally by surprise, he was out cruising (wetlands) in the afternoon, I was not even looking for a snake that day.

[Loren]

I remember finding a Valley Garter (t fitchi) out on a rainy night once. For all I know it was flooded out of its hide spot though. If I was out in the rain, I would say I was most likely looking for amphibians, and it was the colder part of the year- so I doubt it was hunting.
I probably put on a thousand miles a year road cruising at night in prime garter habitats, and this is the only occurance I can think of. No fresh road kills come to mind either.
Doesnt mean they couldnt be active at waters edge, but not likely in my opinion, as at least a few of them would likely find their way onto a road(and then seen by me).
My experience would only be applicable to garter species found in central ca.

[Zephyr]

During the hottest weather in August I SAW a pair of what looked to be male easterns in the shade of a young pine tree in some brush. I saw no other snakes that day.
However, on the contrasting end of the story, I caught a wandering garter in Utah crossing a small field at about noon. All the other wanderers I found were either hanging on the water's edge or in the shade of willow trees, which had to be a good 15 degrees cooler than in the sunlight.