A day seems to be what takes place between 2 reviews (morning and end of day review).
On Sunday Explorers I (Ward Cunningham) showed three projects that appeared over a few hours but were "in production" sufficiently to prove useful and attract additional effort when convenient. matrix 
Programming by experimentation is exemplified by a Beck cycle which is a week long. A Beck cycle starts Monday morning with an idea. Without any planning, a programmer starts coding to see if the idea can be implemented. By the end of Monday, he will have convinced himself of the idea's feasibility. Tuesday, and Wednesday are spend finishing a prototype implementation. Thursday, and Friday are spent showing the prototype to his colleagues, and fixing any bugs that are found. If a colleague likes the idea, he wil l pick up the prototype, modify it, and …