Myriam's got it right: A next action only belongs a calendar if it is tied specifically to a date or time (i.e. it can only be done on this day or at this time otherwise it dies). I'm not saying you can't make an appointment with yourself to work on it at a certain time, but the original reminder must remain in a context list.
Originally Posted by Myriam
I have plenty of day-specific reminders like "Do x today?" or "Good day to do y" if weather or other condtions are right for performing an action on a particular day, but the original action stays on a context list.