Setting Up a Schedule
There is nothing worse than seeing your publication deadline approaching and having no clue as to what your newsletter is going to be about. Planning is essential to making your newsletter a success. You should maintain an editorial calendar, usually at least 2 to 3 months in advance, to identity the subjects for upcoming newsletters. This does not have to be set in stone, as you have to be flexible to take advantage of what's happening in the world. The real advantage to an editorial calendar, besides keeping you on schedule, is to learn to better identify trends so you can write about them or organize multi-part topics in a logical way.
Before you can setup your calendar, you will need to decide how often to send out your newsletter. Daily and weekly schedules are best left to breaking or current news publications. Although they may get glanced at, most people don't have to time to closely read them. I have found that a monthly publication is ideally suited for most newsletter writing. They are far enough apart for people to notice you again, but not so far apart people have moved on to other vendors.
A monthly publishing schedule allows for a more formal, lengthy, detailed, and more carefully researched format. By having a better written newsletter, it also increases people perceptions of you business being a professional and expert in your field. If you want to touch your clients in a more frequent and casual way, then consider setting up a Blog to connect with your readers and expand your site's content.
Finally, I've seen many different opinions on when to publish, but the general consensus is that the best time to publish is in the morning from Tuesday through Thursday. Whatever you choose, the important thing is to be consistent. It demonstrates your reliability.