I'm using Outlook.com's calendar and for whatever reason it insists on showing the birthdays of already deleted Skype contacts. When I try to edit or delete the birthday events I'll get a popup saying either there's been an error or I can't access that calendar right now. This is probably a leftover result of Skype and Windows Live being merged.
I found an answer here, but it was somewhat incomplete. Delete the birthday calendar and then recreate it. When it's recreated it will only list the birthdays of your current Skype and Outlook.com contacts. Unfortunately I couldn't figure out how to recreate the birthday calendar (it's not the same as making a new calendar and calling it "Birthdays"). A little more google-fu turned up these instructions on how to do exactly that.
