Calendar's interface has been condensed a little but otherwise looks the same as it did in OS X Mavericks.

Yosemite
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Mavericks
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The rest of the main window should be familiar. There is a "Calendars" button in the top-left corner of the window that will show/hide all of your calendars. Just to the right of this is a "+" button that brings up a "Create Quick Event" pop window. The standard Day, Week, Month, Year view buttons are still in the middle of the window bar. The top-right corner still has a Search field.

The main area where Calendar sees improvement is the during event creation. Click on the "Create Quick Event" and as you start typing you will see a drop-down list with related events already in your calendar. Typing "babysitter" or "doctor appointment", for example, would bring up a previous entry with the time and location. If you select a previous entry from the list, a new event will be added with all of the same information from the previous event with the exception of the date changing to the current day.


Existing Event
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Quick Create Event
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Newly Created Event
(name, day, address, locations in Maps, the current day, and alert)
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