When I saw the thread title, my first impressions were "Sunday Throat" could be the effect that beer the night before has on your throat the next morning as I often associate Saturday evening as an evening of having a few beverages, and there is an effect on your throat and singing voice the next morning. Or it could have been a religious expression as Sunday in Christianity is the Sabbath and it could mean singing your Sunday best like being dressed in your Sunday best. I molded the two together, as when I drank, three or four beverages in the evening would make my voice sound like my Sunday best the next morning. That was my idea before I read the thread. I thought it would be a creative theory.