Looking at the Alexa stats for Wikipedia: http://www.alexa.com/data/details/traffic_details?q=&url=wikipedia.org
you see some interesting patterns. The graphs seem to have a sawtooth shape with a weekly period, but the specific days of the week where it peaks differ depending on whether you're looking at reach, traffic rank, or page views.
Raw page views seem to peak on Sunday and are lower at other times of the week, though there's a smaller second sawtooth around Wednesday. However, the reach statistic seems to be lowest on Friday and Saturday and peak around the middle of the week, with Sunday being an increase from Fri/Sat but not a major peak. The traffic rank follows the reach statistic, dipping on Friday and Saturday and holding steady at its highest level the rest of the week.
Can anybody venture any theories (original research!) about why this is the case?