If you have a lot on your plate — and who doesn’t? — alerts can be a big help. Your iPhone beeps (or vibrates if the Ring/Silent switch is set to Silent) and sends a notification message at the interval you select, from five minutes to two days before your event.
Even if your iPhone is sleeping and/or locked, Calendar wakes your iPhone if you turn on Show on Lock Screen in Settings→Notification Center→Calendar. You hear the beep, or whatever sound you choose for Calendar alerts, and the notification appears. Slide the notification to view the event.
If you are actively using an app, a banner appears across the top of the screen at the appointed alert time. When you sync your calendars, alerts sync to the corresponding calendar on your computer, and vice versa.
You can even receive two alerts, so you can be reminded of your dear Aunt Sybil's retirement dinner two days before the event, giving you time to get a gift, and then again, the day of the dinner. Follow these steps:
Tap Alert on the Add Event screen or the Edit screen, if you want to add an alert to an event you already created.
Tap how long before the event you want to receive an alert.
You return to the Add Event screen, but another field is added under Alert, the Second Alert field.
Tap Second Alert if you want to receive two alerts for the same event.
Tap how long before the event you want to receive a second alert.
You return to the Add Event screen.
Tap Done.
The Notification Center Settings work with Calendar in two ways:
View Calendar events in the Notification Center by turning on Settings→Notification Center→Calendar Day View and/or Tomorrow Summary On.
Choose the alert sound and style in Settings→Notification Center→Calendar→Calendar Alerts. Tap the sound you want to hear or None for one or both types of alerts if you don’t want to receive a visual or audible notification.
You can set default alerts for events or all-day events by tapping Settings→Mail, Contacts, Calendars→Default Alert Times. Then tap Events or All-Day Events and choose when you want to receive an alert.
This gives you an alert for all of those kinds of happenings, so if you have a lot of events, setting up default alerts may be more a cause of confusion than a reminder. In that case, you may want to only assign individual alerts to your most important events.
If you have an alert scheduled during a time that Do Not Disturb with the Always option is turned on, you won't receive the alert.