FullCalendar is a popular jQuery plugin that provides a full-sized, drag & drop calendar. This plugin is updated now with Version 2.1.0 that  uses AJAX to fetch events on-the-fly for each month and is easily configured to use your own feed format. It is visually customizable and exposes hooks for user-triggered events like clicking or dragging an event.

jQuery Plugin for Full-sized Drag & Drop Event Calendar

Demo Website