Non-posting days are like buses - you wait for ages then two come together.
I'm going through a patch where I just don't have anything to write. But that's okay. Better to not write and be comfortable with it than post rubbish.
The likes of Seth Godin turn up every day without fail but it's not for everyone. It's actually not for a lot of people.
There's an art and commitment to constant output - sometimes we just can't support it but that's nothing to be ashamed of, whether we miss the odd day or don't post for ages.