Office 365 (Outlook 2013 | 2010) - Create an appointment from an email message
Click on the message you wish to associate with an appointment/event.
Holding down the left mouse button, drag the message to Calendar in the lower left-hand corner of the email client. Once you have the mouse hovered over Calendar, release the left mouse button.
This will open up the Appointment window. You can now customize the date, time, and location of the event. When finished, click Save & Close in the upper left-hand corner of the window.
Note: Any attachments from the email message will not transfer over to the appointment. You will need to re-attach them.
The appointment will now appear on your calendar during the date and time which you specified.