Advent Joy differs from the “everyday definition” of joy more in it’s subject than anything else. It’s not just being happy about stuff. Christian Joy is about how wonderful God is, how wonderful the things He has done. And how wonderful His promises are.
The cool thing about this kind of joy is that the object of it — God’s amazing character — doesn’t change. When you’re joyful about your circumstances, guess what? When your circumstances change, your joy will go with it. When you’re joyful about God, you can always go back to that cause for joy.