"In that day you will say: 'Give thanks to the Lord, call on his name; make known among the nations what he has done, and proclaim that his name is exalted.'" NIV translation
Now, we see the other song and its purpose is to tell those that do not know Him about Jesus and what He has done for us and them.
"'Sing to the Lord, for he has done glorious things; let this be known to all the world. Shout aloud and sing for joy, people of Zion, for great is the Holy One of Israel among you.'" NIV translation
Joy; that is what the children of God should be known for. Whether it is in good times or bad, the world can and should see the joy of our Lord through us. We can be unhappy because of our circumstances but our joy is in the Lord and therefore does not depend on what is going on around us.