In Mountain Lion, Apple split off its reminder options from the provided calendar program and put them into a separate program similar to the Reminders app in iOS. This program lets you keep track of your projects, routines, or what have you, with to-do lists for each. As you make progress, you just mark each reminder as completed, and it drops off of its respective list.
But if, say, you inadvertently mark the wrong reminder as completed, don't worry, you can still get it back.
The Reminders program does have a Completed section that you can click to see … Read more