Projects can contain an unlimited number of days which share the common properties of their projects. Days are used for the actual entry of call volumes or agent numbers. In practice, you would set up a project to model the operation of a call centre or of a particular campaign. Each day you assign to a project represents different call volumes and patterns for that call centre or campaign. For example, if you know that incoming call volumes on Monday are much higher than on other days, you could set up one day representing Monday, and another day representing Tuesday to Friday.
In this way, you can analyse the requirements of a call centre subjected to different levels of incoming calls, but sharing properties common to a campaign, such as average call duration.