Two days ago my "Service Engine Soon" light came on and I was going to get the codes read to see what's up. Today when I started the van the light was out and it did not reappear. I was surprised as I thought the light stayed on for a many cycles before it would go off by itself.

Can anyone tell me more about why the light would go off without anything being done. Is there still a code that can be read even though the light is now off?

There should be a code stored because the light did come on.
It my end up being a driveability problem such as a loose hose or a sensor may have not operated properly momentarily, enough to trigger the light. As soon as you can check for stored codes either at a place that checks them like Autozone, an independent garage, or if you already have a code reader you can check it yourself. From my experiences the ones that occur like this are usually minor issues, easily fixed and cheaper to "nip in the bud" before they can get serious.