If you need a quick instructions on how to add a calendar that someone sent to you:

If you haven’t used a calendar with HotMail, you can either try cut and pasting the webcals link above into your browser, but if that doesn’t work then use this process

Open Hotmail and get the URL from your message

image Select Calendar from the left side
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Now add a new calendar

image Click Subscribe
   
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Add the webcals link above to the subscription, you could save the ics file or use it directly as shown

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Once you have everything done correctly then your calendar sidebar should look like the image to left