The most common single day was Tuesday (which just so happens to be the day when new music hits the iTunes store, presumably meaning more visits who might see a new podcast).
To find this conclusion, I pulled the numbers for the Top 25 podcasts in the iTunes store and noted their publishing schedule and the frequency with which they published new podcasts. There was a large variety of posting schedules among the Top 25, but a small trend did begin to develop. Sixty percent of podcasts with a regular schedule posted early in the week, before Wednesday. The most common single day was Tuesday (which just so happens to be the day when new music hits the iTunes store, presumably meaning more visits who might see a new podcast).