Spotted...

Most cyclists commuters ride in 2-3 times a week (I've done a thorough research using my colleagues as guinea pigs).
Mondays and Wednesday always seem to be the busiest days.
Thursdays, people go out in the evening and use public transport.
Fridays are usually super quiet, people go out again, have days off etc... less traffic all round. I like Fridays!

Also, from my experience, Wednesday sees more Peds about in Londons famous Londons West End as it (might be) cheap night at the theatre.