Sunday is the best for my area in Washington, DC. 1-4 PM is generally the best time. That's when people are out looking specifically to find a place.
That being said, Saturday can be a great day to pick up people who aren't shopping around.
Also, working some holidays can be great like Easter etc. Families are in town and might like looking at houses together. While other agents might not hold it open for that reason, having yours open can get special exposure.