To display those results, Calendar excises the reminders sidebar; as Mountain Lion includes a dedicated Reminders app, you're expected to manage tasks and reminders from within that program, instead. Of course, this also means that if you want to create any sort of to-do, you need to hop over to Reminders to do so; unlike with Contacts, there's no direct link to the Reminders app in Calendar.
The other major change in Calendars is that Apple has completely revamped alerts and event reminders. For each of your accounts, you can now customize your default alerts, individually, for timed events, all-day events, and even birthdays; each of these can be set to occur a chosen number of minutes, hours, or days before the event.
You'll also find a few minor interface changes. For example, the Calendar menu is gone, with its commands redistributed among the File, View, and Edit menus. And you can now disable the "heat map" display for Year view.
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