I usually think of joy as happiness that goes deeper than a good feeling you have on a day where everything goes exceptionally well. Henri Nouwen backs this up. Happiness depends on what's going on around you. Joy is knowing that you are loved without condition or restriction. Nothing can take that love from us. God is the source of that love.
If you're getting a divorce or newly diagnosed with illness, you can still celebrate joy. If you're alone or you've been sick for a long time, joy is still yours for the taking. If you're sad, joy can still be how you choose to live life.
As the days get shorter and Advent comes closer to its ultimate celebration, remember to look for the joy in everything you do and see.