I guess it would depend if your tagetting a certain region of the world and also if you believe that the majority of your readers may be reading from work/school,university.
What Pipes said as well as niche within that region and what you are trying to get from your visitor, a read, subscription, purchase?
I think the middle ofthe day in US is best. Alot of people read blogs while at work and shop online at work.
It may and may not matter too much when you post, as if people are interested in what you have say then they will come back to read it regardless of what time you post. You could post anytime of the day, but I'm sure you already know, there is no point in posting if you content is not good quality and of use to the user.
Sunday, and Thursday for me. I always save good updates or releases for those days....I tend to get more traffic then.
Well you have two different time zones there as a minimum, east coast USA is i think 6 hours behind UK and west coast is behind UK 8 hours, it depends what your writing on but as mfolan pointed out it may not matter as people will come to read it when they can, even if they are reading a post a week later.
I'd say the reason for your higher traffic on SUnday and Thursday is because they are the days that you post and people know this and come back on these days. It may be that if you posted another day regularly you will have increased traffic on that day too.
According to the image at the link below there are 4 different time zones in the states. www.tandilnet.com/Turismo/zones.gif
I am not very sure how we can time the post, to maximize the readership. I think, all it matters is to be consistent. I post on my personal blog, quite sporadically. But on my art blog, I post at 12:00 GMT every Sunday and Wednesday. The time is quite arbitrary - but I am alway regular, making use of the 'future post' option of the Wordpress. But I am open to exploring the good times. Owing to the fact that my blog is an Art blog, and I find international visitors - I am not sure how I can time the posts. It does have a heavy percentage of visitors coming from US and UK.