The most dangerous day on US roads in 2010? October 16, when 204 vehicles were involved in 90 crashes. Compare that to a low of 41 crashes on March 22 -- and to the rest of that year, with this illuminating visualisation.
There were 45,777 vehicles involved in fatal crashes in the US during 2010, according to this clean, informative, and sobering chart from Flowing Data. It's a rich pool of data showing exactly how many accidents happened on every day of the year.
Weirdly, there are fewer crashes in winter. Despite worse conditions, people spend less time on the roads, so fewer accidents occur. Weekend driving is a bad idea all year round, but particularly over the summer; that's when most of the accidents happen.
You should definitely check out the full visualisation by clicking on the image on the right, because it makes for fascinating reading. One interesting stand-out data point is New Year's Day, which has easily the highest number of accidents for the whole US winter. [Flowing Data]
Image: Flowing Data, Mr Wabu</a.