Our circumstances may change from day to day, but our response doesn't: each day we rejoice in the Lord.
I admit, I find it easier to rejoice some days than others. Like today--it's sunny, I'm looking forward to a movie night with my family, and everyone in my life is healthy. Pretty easy to rejoice.
But we can also rejoice on the hard days, the cloudy days, the sick days. Rejoicing doesn't mean we'll never be sad or overwhelmed, that we'll never grieve or hurt. But it does mean that even in those times, we have the joy that comes from knowing that no matter what happens to us in this world, God has promised us eternal life with him in heaven--where we will never stop rejoicing.