Bloggers should always have a clearly outlined posting frequency. It does not matter if you post once a week, three times per week or every day as long as your readers are aware of that (in reality I advocate that if you are serious about blogging you should write at least 5 weekly posts, but that is not the central point of this article).
The problem is that most people have other activities that might disturb the normal posting schedule. Your family might need your attention certain times or you day job might require some extra hours of work on specific periods of the year. The best solution for those emergencies is to have some posts already written, just waiting to be published.