Great question. I have noticed that, as the real estate industry has evolved over time (and especially as more people's schedules have become extremely over-crowded), home buyers have (quite understandably) adapted an "I want it now" mentality. So, instead of planning their weekends around going to Open Houses (when it is convenient for a seller), they have an expectation that they should be able to view homes on their timetable. So, the short (and most honest) answer to your question to: when is the best time/ day for an Open House is usually: never. However, if you want to try doing Open Houses as a means of marketing a home, I recommend offering buyers a relatively small window of time to be able to come see the home (as opposed to several hours). The reason for this is: you want to create "demand" for the house. Shorter time windows create a situation where, if people do come to an Open House, more people will be there at the same time. The perception this creates is that there are a lot of people interested in the house, and if they are at all interested in buying it, they need to move quickly and make an offer (as opposed to most "traditional" 2 or 3 hour Open Houses, where 1 or 2 people might "straggle" in at a time, which creates the impression that there is no need to act with a sense of urgency. It tends to it make it seem as if no one else is interested in the house.)
* This message is not intended to solicit currently listed property.