It all depends on how one measures it. If five days are considered a week, then it is not the worse five days ever for the Dow. From Tuesday, July 18, 1933, to Saturday, July 22, 1933, the Dow dropped 18.33%. If this week is compared with any other Monday-Friday or Monday-Saturday, it is the worst week for the Dow ever. I heard on a web video, however, that for the S&P 500, that same week in 1933 was slightly worse than this week. However you look it at, it was really bad.
How does the generally trend over the past 11 months compare with other bear markets?
http://www.nytimes.com/
Also, try this on the five year setting (pretty scary looking):
Yahoo chart