I find this question funny , since I have been pondering the same thing for a few days now.
Most of my readers are from the US, so I find it useful to post in the morning during workdays (it seems many of us are spending too much time on HP besides doing our work..I do).
On weekends and on some days I post at night, like now but I'm, way too tired (it's 11:04pm) for the benefit of my overseas readers.
Try this: write two hubs and post them on the same day one in the am and one in the pm and check visitors. This may give you some idea. Note; both hubs must be similar in content and in length.
Right now I have several unpublished hubs on which I am trying this.