This is another very productive feature of Outlook.com. Normally you use at least one calender in your Outlook.com account. You can upload other calenders to your account. For example I have total 5 calenders in my Outlook.com account (work, birthday etc). When I log in to my account and go to calender, I can see all my calenders and events/meetings inside them. Now world cup cricket is going to start this week and I want to follow India matches. What I do? I went to ICC website and download a calender including all the India matches. Now I can attach this calender to my Office Outlook or upload to Outlook.com. After successful uploading I can get reminders of India matches like any other event/meeting in my computer or phone. You can share this calender with your friends also.
I find this little feature is very helpful. If you are using some other provider like Gmail, you can do this in same process.