When the list is compiled, the material day is the day the list is compiled (1 April 2005 for the 2005 valuation list). If the list is further altered, the material day may be different for that alteration. For example, in the 2005 rating list the material day for an alteration to reflect an extension to a property will be the day the extension is completed. Previously the material day for an extension had been the day on which the list was altered. There are a number of different categories of events which cause the list to be altered (such as a new entry to the list, or the merger of properties), each of which has a specified material day. Most commonly the material day is the day the event occurred.