You mean an actual Calendar control, or the built in date picker that shows up at textboxes when they're based on a date (provided your option settings allow)? If the latter, a simple AfterUpdate event should do the trick. Question is, is the form (along with that control) bound? If so, requery the main form in the event. If the control isn't bound, you requery the control. The subform should reload data based on the textbox value being changed (assuming that's the common link).
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.