Weekly Cycle practice is a recommendation for reducing the short term planning period to one week. When developing software in the evolutionary way the architecture and design of the system under development are supposed to change. The high level decomposition usually stays intact for a reasonably long time, while low level details can get outdated often. The weekly cycle practice takes this fact into account and recommends for planning in detail for no longer, than a week.
Weekly cycle is especially useful, because it matches the natural work cycle. Many teams find it effective, because it makes everybody focused on Friday and lets people go for a weekend after accomplishing and demonstrating all the stories planned. However, some teams prefer starting and ending the cycle on Tuesday or Wednesday in order not to have planning on such an "unpleasant" day as Monday. That is OK as long as it does not force people to work during the weekend. Many teams find it useful to start the weekly cycle by writing the acceptance tests for the selected stories during the first couple of days and spending the rest of the week making the tests pass.
This page is a part of the Extreme Programming overview