How do repeating / recurrent ToDos work?
Repeating ToDos can either be on Due Date or Completion Date. Every time you need to use the 'Repeat' feature, you also need to set a Due Date (as it cannot repeat without being due on a date to begin with). When you set a recurrence pattern on a ToDo, it's due date (if not set) automatically gets set to 'today'. You will now only see a single task in the list that's waiting to be 'Done'.
Say you create a new task and set its recurrence pattern to every Monday and every Tuesday. Once you complete the task, a new task is created with its due date set to a 'Monday' or a 'Tuesday'. Here's the tricky bit: If you had set it to repeat on 'Due Date', and had a due date of 1st Dec 2009, it would automatically set the next due date to 7th Dec 2009 (which is a monday)' even if it was the 29th of Dec 2009 today. Then when the task is marked as Done again, it would move forward to the next Tuesday from the currently set due date and so on.
However if the task was set to repeat on its completion date, it would move forward in time as of 'today' (since you would be marking it Complete 'today'). So if today is the 29th of December 2009, the next Due Date set for the task would be the 4th of Jan 2010 (Monday). If you then mark this task to be Complete again, it would move forward from its current due date (i.e. 4th Jan 2010) to the next Tuesday (which is 5th Jan 2010) and so on.