To enable Drag-and-Drop Scheduling Go to Setup-> Customize -> User Interface -> Go down to Calendar and Check:
Enable Click-and-Create Events on Calendar Views, and Enable Drag-and-Drop Scheduling on List Views.
After your administrator sets up drag-and-drop scheduling, you can click the Open Calendar link at the bottom of a list view to display a weekly view of a calendar underneath the list. Then, you can drag a record from the list to a time slot on the calendar to quickly create an event associated with the record. When the calendar is open, you can:
Select the Change link to switch to any calendar to which you have access
Click the Previous Week () and Next Week () icons to view the previous or next weeks in the calendar
Click the Date Picker icon () to choose a specific date on which to view the calendar
Select the This Week link to view the current week in the calendar
Select the Show Weekends checkbox to display weekends on the calendar
Click the Close icon () to close the calendar at any time. Alternatively, you can click the Close Calendar link at the bottom of the list view to close the calendar.