I know exactly what you mean, and most of the time I catch myself thinking that it's on a weekend. Possible explanations:

1.) The people that post more technical stuff do so from work and don't use HN as much on weekends.

2.) Everyone is in "weekend mode" and wants to post more fun stuff.

3.) I am in "weekend mode" and my desire to click on fun stuff is biasing my impression of what is posted.