Generally it's the entire weekend1 for the internet; however, depending on your site it could even be a Wednesday that produces the least traffic.
In my case the weekends are always dry, and especially so if there's something like the Oscars on air (drat...). My stats are too poor to put up for display here, but yes, events of note do woo away your readers, you can take my word for it. I think I'll cut down on my weekend blogging...
1The figures provided in this link are from 2004, but unfortunately there doesn't seem to be any recent statistics on this.
