Internet Calendar (iCalendar)

In Outlook, when you want to invite people to attend a meeting or event, you usually send a meeting request. The recipient can then accept the request and the item is added to his or her calendar. Alternatively, you can add an Internet calendar (iCalendar) attachment or a link to an iCalendar file that works more broadly with Outlook and other calendar programs to the message body. When the recipient opens the attachment or clicks the link, the event is added to his or her calendar program. 

Note: Unlike Outlook meeting requests, when using iCalendar attachments and links you do not receive confirmations when people accept or decline.

Sender: Create an iCalendar link in an email message

Step 1: Create an appointment

In Calendar, on the Home tab, in the New group, click New Appointment to create an appointment.

Step 2: Send an iCalendar attachment

On your calendar, click the appointment. On the appointment tab, in the Actions group, click the down arrow on the Forward command to Forward as iCalendar. A new message window appears and an iCalendar is attached.

Recipient: Add and import calendars

Step 1: Download internet calendars

Right-click the iCalendar .ics attachment, and then click Save to save the file to your computer.

Step 2: Import the iCalendar file

In Outlook, select File > Open & Export > Import/Export. 

In Import and Export Wizard box, select Import an iCalendar (.ics) or vCalendar file, and then Next. Select the calendar file from your computer, then Import to import the item into your calendar.

Source: Create a calendar link