You can have a respondent put the event in their calendar by linking it to an ICS file. This is a general format for calendar items and can be opened and saved in calendars such as Outlook calendar, Gmail and iCalendar.
So, how do you offer such an ICS file? First, open your own Outlook calendar and create a calendar item for your event. Save this as a .ics file using Save as. Copy this file into the Messages module and create a link to it from the confirmation email.