Once a plan was set, subsequent shopping would be infrequent and subsequent workplace decision making would be without the hassles of catering to bosses. Much of what we seek to consume will be delivered to council outlets or even communities. Imagine the time that will save. But what we want each week will continually change. Even if your slack and update techniques allow plans to accommodate the whole population, regarding weekly deliveries I certainly won't be able to decide in advance what I want every week unless I know when I'm going to have guests, when I'm going to eat out, and so on, a whole year in advance.
Nobody knows that.
That's true, so in our society we will put in orders for food deliveries on a monthly or weekly basis, whatever makes sense. And even though your order may fluctuate each week, it is averaged with the orders of everyone else in your neighborhood, so shippers are less likely to be surprised by bulk orders. The whole idea of averages allowing localized fluctuations with minimal inconvenience to producers works rather similarly in market and centrally planned economies. For example, imagine that in a market economy everyone unpredictably decides they want twice as much milk, or shoes, or whatever. They would encounter delays probably worse than in participatory economies because the participatory model has much better communicative facilities.