In Calendar Options, you can specify a default that automatically sets an alarm each time you accept an appointment.
Click Tools > Options.
Double-click Calendar, and then click the General tab.
Select Appointments and/or All-Day-Events in the Accept Options box.
Set the amount of time before the appointment and/or All-Day-Event the alarm should notify you. The default is 5 minutes for appointments and 12 hours for All-Day-Events.
Click OK.