I was never a big Chicago fan, but band friends were thrilled that brass had made it big in Rock 'n Roll, and they were needed to perform these songs right. Few bands had brass players who could manage this, so they had a short run of being much in demand. "Blood, Sweat, and Tears" and "Earth, Wind, and Fire*" songs were also done by those bands.
For years I missed what the reference was to in the song title, and found it merely puzzling. "Waiting for the break of day" was apparently not enough of a clue for me. It was an age of overinterpreting songs to find drug, political, or religious meanings in them, while neglecting the obvious in mildly poetic language.
*What fun those two groups would have been in concert, eh? "Blood, Earth, Tears, Fire, Wind, and Sweat." Or something.