Q:

Display a Notification Once a Week

I am new to notifications in Android and I want the app to display a notification once every Sunday.   I am not sure how to do this when the activity is not running. 
Thank you in advance! 

A:

You should start reading and understanding how alarms works: here then you can make the alarm activate the notification on given time. 
NOTE: that the alarms won't get saved after reboot so you will need to add a boot complete receiver.