Please note: This article relates to Outlook Calendar for Desktop. If you are using a different calendar this article might not be applicable.
When you load an appointment into Appointment Reminder you need set the time before an appointment when the reminder will be sent. (most people use 24 hours)
You'll want to set the default "when to send" time to the most common time you use. This will save you having to edit this every time you create a new reminder, also reduces errors.
In Outlook click the Appointment Reminder Tab at the top. Click Settings. Enter your username and password. Click "Check Credentials" and then change the "Default Send Time" and click save.