The "us" is important. Everyone in that congregation had known some kind of pain or rejection. Yet, they could rejoice.
But, even in that rejoicing over what had been overcome, we still need the strength and support that God gives us. This is the day. Every day is this day.
You are my God, and I will give thanks to you; you are my God, I will extol you. O give thanks to the Lord, for he is good, for his steadfast love endures forever.