If you’d like to temporarily stop alerts and banners in Mountain Lion (MacOS X 10.8) Notification center, you can quickly turn them off without having to go in and disabling everything, and it only takes one click:
Hold down your Option key while clicking the notification center icon in the top right of your screen:
That’s it! You will see the icon change to a ‘grayed out’ version. Notifications are now off.
To turn them back on, you can either option click the same Notification Center icon again
or click the icon and toggle the “Show Alerts and Banners” settings to “On”.
You can also just wait, because they will auto-resume the following day.