Open houses are almost always on the weekend. But "which day" is better/more productive is almost always up for debate and even a potential argument, depending on who you're talking to. A few days ago a buddy of mine got himself into a mouthy conversation about holding open houses w/some fellow Realtors friends of mine.
Personally I prefer to hold an Open between 11-2 on Saturdays, and never on Sunday's. I don't like competing with church or the NFL. Many people consider Sunday the Lord's Day and that means church, family time and mostly relaxing, not working or driving around to open houses. I work, play and enjoy my weekends, so either day for working is fine for me. I believe you take advantage of employment opportunities 7 days a week. But that's just me. I simply see Sunday's as less productive for traffic.
What day has been more productive for you as a Realtor?