This bit about Apple’s new version of OS X. Emphasis mine.
Apps have been renamed for cross-OS consistency. iChat is now Messages; iCal is now Calendar; Address Book is now Contacts. Missing apps have been added: Reminders and Notes look like Mac versions of their iOS counterparts. Now that these apps exist for the Mac, to-dos have been removed from Calendar and notes have been removed from Mail, leaving Calendar to simply handle calendaring and Mail to handle email.
Well how about that.