You can have todo items show up in the Emacs Fancy Diary display by
including the todo file in your diary file (see (emacs)Fancy Diary
Display). This effectively augments the Emacs diary with
categorized diary entries. All items in an included todo file will
appear in the Fancy Diary display except for those that are marked
with todo-nondiary-marker. You can add or omit this marking
upon creating a new todo item, or you can do so by editing an existing
item, see Inserting New Items and Editing Item Headers and Text for details.
To ensure the proper display of todo items in the Fancy Diary display, they must have the format of diary entries, i.e., they have to begin with a date string recognized by the Emacs diary,1 and if they are longer than one line, all lines but the first must begin with white space. Todo mode ensures that these requirements are satisfied (see Other Display Commands and Options).
The Fancy Diary display is also Todo mode aware: if it contains an item from a Todo mode file, clicking or typing RET on this item will switch to the buffer visiting that file and properly display the item’s category, with point on the item.
Two types of dates recognized by the Emacs diary are not supported in the current Todo mode implementation: sexp diary entries and date strings in which the year is omitted (however, the latter type is equivalent to using ‘*’ for an arbitrary year, which Todo mode does support).