I agree with bl4ckman, it depends on niche... My earning gets doubled during weekend... Tuesday & Wednesday are my slowest days...
As far as clickbank goes, for me my slow day is every single day of my life except for the 2 days I've had sales... And as for the slow days traffic-wise, One of my sites gets about 1/10th its usual daily traffic on Saturday and Sunday... combined.
depends on the niche. For the most I believe it would be Friday and weekend. It would be interesting to see a thread on the matter with a poll...