Notifications and alerts have been given a big boost in OS X Mountain Lion and there are more ways to be alerted than before. This is mainly due to Notification Center and iCloud syncing across computers and devices. The number of alerts you see and when you see them can be customised, so let's take a look at Calendar.
Start Calendar and click Calendar, Preferences. Select the Alerts tab. There is an Account selector at the top and you will have an iCloud account (who hasn't), and possibly one or more others. For example, you could also have a Google Calendar account too. It is possible to have different alert settings for each account, so don't forget to customise them both.
There are three types of Calendar event and there are birthdays, all day events and (other) events. You might want to be alerted to birthdays a week before the even so that you can go out and buy a present or a card, or you might simply want to be alerted on the day so you can send them a birthday message or post on their Facebook page.
All day events have the same options to birthdays and you can be alerted on the day, one or two days before or the week before. Other events tend to be meetings, appointments and so on and for these there are options to be alerted 5, 10, 15 minutes before, one or two hourse before or even the day before. It is hard to find the right setting for these because an event could be a meeting with work collegues in the room down the corridor that takes 30 seconds to walk to or a meeting with a customer in a different city that takes hours to get to. It's up to you what setting you choose.
If you use more than one Mac, such as an iMac and a MacBook, or if you have a work and home computer there is an option to have different alerts on different Macs. On your home Mac for example, you might want to be alerted to meetings the day before they are to take place. So if you check your Mac the night before you'll see an alert making you aware of meetings taking place the next day. Your work Mac could be set to alert you an hour before a meeting. This can be quite useful for some people. Tick the box Use these default alerts only on this computer. (This is only available on iCloud calendars and not others, such as Google.)
The alert settings in Calendar Preferences described above are default ones. In other words, if you just create an event in the calendar and don't bother to set any alerts, the default will be used.
General settings are OK for birthdays and all day events, but as pointed out above, there isn't a single setting that can be applied to general events like meetings and appointments. You will probably want to customise the default alert or even to add additional alerts.
Click in the calendar to to create a new event, then double click it to edit it. Clear the all-day tick box if it isn't an all day event. Two alerts are listed and the first is set to the default.
Click the first alert and you can change the default to something else, so if you need to travel to a meeting, set the alert to one of the options, like Message, in the pop-up list and then below another pop-up list appears in which you can select minutes, hours or days before.
You could have two or more alerts as shown in the screen shot, so you could change the first from the default so that you are alerted a day or two before an event and then set the second to hours before (it's fixed at two).
This causes a third alert setting to be displayed and you could set this to minutes before (it's fixed at 15). This way you can be sure never to miss a meeting or appointment.
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