The 13-period calendar splits the year into 13 four-week periods rather than 12 months of varying length. A 13-period calendar is useful for accounting purposes, such as comparing sales figures or scheduling pay days. 

Google Calendar standard offers the option to add a week number (links to a page in Dutch) to the calendar. The period calendar uses the same approach. Add the calendar ID in your Google Calendar via "Add" and "Add co-workers calendar" or use the URL via "Add" and "Add by URL". 

Receive the instructions for this calendar by mail by filling in this form. The mail contains the information on how to add this free calender to your personal Google Calendar.