I post M-W-F around 4:30am PST and typically traffic is much heavier at the beginning of the week. I do post bonus stuff on Saturdays from time to time and the readership rate is about the same for any other day I post.
I think a lot of people read while they're at work or school. There are some very popular comics that only upload on the weekend (Glacias, Suihira).
If you only post once a week, you could experiment and try different days/time. Just make sure you are consistent in your uploads.