"This is the day which the Lord has made.
We will rejoice and be glad in it."
That has oftentimes been sung as a song around the campfire or in Sunday schools. It's usually taught as a song about "ain't this a nice day that God gave us" or a "cheer up and be happy" kind of song.
But when you look at it in context, the day is the day when the Stone which the builders rejected became the Chief Cornerstone. This is a Passover psalm. This is the psalm chanted on Palm Sunday by the crowds welcoming Jesus. This is the psalm we still sing in the Sanctus every Sunday.
If we actually understood this campfire song, we'd realize that this song isn't a "cheer up" song, but is really telling us why Good Friday is good.
"Oh, give thanks to the Lord
for He is good,
for His mercy endures forever."