According to an article by Business.com, the best day of the week for driving traffic to your website/blog/whatever via Twitter is Monday.
They also found that there are specific times during the day--windows--that garner the most twitter activity. As they say:
"Our tweets with the highest click activity were posted in one of three periods:
- After 5pm PDT / 8pm EDT - when people on the East Coast may start getting back to work after an early dinner or putting the kids to bed
- 11am-noon PDT / 2-3pm EDT - when attention wanders in the mid-afternoon on the East Coast and before lunch in the West
- 5-7am PDT / 8-10am EDT - when people first start sitting down to work and catching up on email/blogs/Twitter before meeting start for the day"