It is not a schedule, it's a standard. One I normally try to exceed. We ship when things are ready, which for my current team is ~2-3x/week, but in the past I've had teams that were faster.

We know that there will be things to ship because we try to break the work down into small units of deliverable value by focusing on seeking the highest-value things to do. Large requests are typically composed of a bunch of things of varying value, so we split them until we find things that advance the needs of the user, the customer, or the business. One that's often not intuitive to people is the business need for getting information about what's truly valuable to some part of the audience. So we'll often ship a small thing for a particular audience and see how they react and what actually gets used. (You can save an amazing amount of time by not building the things nobody would end up using.)

Sometimes we can't figure out how to break down something smaller enough that we have something to release right away. Or sometimes a chunk of work surprises us and it drags out. We avoid that, because compared to working on smaller things, it's much less comfortable for both developers and business stakeholders. But if it happens, it happens. We try to learn from it for the next time.

Regarding deadlines, we sometimes have them. Broadly, efforts break down into two categories: driven by date or driven by need. For the former, releasing early and often means we adjust scope to hit the date. For the latter, scope is primary and they get stuff when they get it. Either way because the business side sees steady improvement and has fine-grained control over what gets shipped, they feel in control.

This can be a learning experience for business stakeholders used to waterfall-ish, plan-driven approaches. But I have never once had somebody successfully work this way and want to go back. I have, however, had some product managers get thrown back into document-driven development and tell me how much they missed working like we did.

No, shipping code to production on a weekly basis is not a deadline. A deadline is a time by which a task must be completed. A task is something like "fix bug 3831" or "allow users to log in with OAuth2". "Ship code" is not, in any useful sense, a task.

Such "timeboxed iterations" can indeed result in "shipping" a null update. Unless you have a time-consuming QA gate to pass, that's not very likely, especially on a team containing several people, but it can happen. You don't know that you will have "something" to ship.

Typically we try to break changes down into shippable tasks that can be done in under a day, so the expected number of tasks completed by a four-programmer team in a week is on the order of 30, or 15 if you're pairing. For this to fall all the way to 0, everybody has to be spending their time on things that could not be thus broken down. It's pretty unlikely to happen by chance. But sometimes a difficult bug or two really is the thing to focus on.

In XP, estimates are used for prioritizing which tasks to work on and which tasks to break down into smaller tasks. The "product owner" is supposed to choose the tasks that have the most user value for the estimated cost. But those estimates aren't commitments in any sense; they're guesses. Sometimes tasks take more time than estimated; other times, they take less. This is the reason for the shift to estimating in "story points": to prevent the estimates from being interpreted as referring to a period of time.

If someone in your organization is interpreting estimates as commitments, this can have a corrosive effect on the accuracy of those estimates, because estimators respond by padding their estimates, in inconsistent ways. Often this destroys the usefulness of the whole iteration planning process, because the project owner no longer knows which tasks are the easiest ones and thus worth doing even if the benefit is marginal. Organizations can recover from this pathology in a variety of ways, but often they don't. Eliminating estimation is one response that sometimes works.