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I suspect during the week the higher bid ads ads show first and burn thier money to be replaced with the less paying ads as the day progresses.
Are advertisers actually paying more for weekend slots believing (my theory) that retail economies are getting back on track and users are doing this over a weekend and not from work?
The weekend clicks could be catch-up click dumps from the prior week.
Which is your theory - that retail economies are getting back on track or that advertisers believe retail economies are getting back on track?