Drupal 7 add calendar module. Doubleclick on a day to add calender item for that day
The Calendar module does not provide such a feature per se. There is also another calendar module, FullCalendar. This uses the FullCalendar jQuery plugin. By default the Drupal module does not provide this functionality, but from the demo pages it seems that it is a relatively easy one to implement by yourself. You would have to study the demo a bit.