gnus-diary provides a function called
gnus-diary-check-message to help you handle the X-Diary-*
headers. This function ensures that the current message contains all the
required diary headers, and prompts you for values or corrections if
needed.
This function is hooked into the nndiary back end, so that
moving or copying an article to a diary group will trigger it
automatically. It is also bound to C-c C-f d in
message-mode and article-edit-mode in order to ease the
process of converting a usual mail to a diary one.
This function takes a prefix argument which will force prompting of all diary headers, regardless of their presence or validity. That way, you can very easily reschedule an already valid diary message, for instance.