How to set alerts in Mac OS X Mountain Lion

Mountain Lion enables you to set notification alerts to remind you of upcoming birthdays, tasks, meetings and other important events you might have in your calendar.

Open Calendar on your Mac computer.

In the menu click Calendar and choose Preferences...

Now click the Alerts.

Choose the account associated wit the calendar you want to change the events settings and set the amount of time you want to be reminded prior to the event

If you want to be reminded 15 minutes before the event in your calender then choose 15 minutes before. If you want to be reminded to buy a birthday present 2 days before the birthday then choose 2 days before, well you catch the drift.

Just close the Alerts window when done.

Now let's test if this thing works.

I'm going to enter a new event in my calender that will start in about 15 minutes ...

... and I got an alert notifying me, I have a New Event (I didn't name my event so by the default it stays New Event) which is great as I've set the alerts to pop-up 15 minutes before the event.