What's so fantastic about this day is it was the moment when Jesus revealed his true identity as king to the people. He'd been keeping it quiet, only revealing it through parables and to his closest friends until then. They had been believing what he told them and showed them. On that Sunday, even his best friends didn't put two and two together. They didn't realize that he was fulfilling an event prophesied in Isaiah and Zechariah. The summary explains that it wasn't until reappearance three days after his death that they put it all together. Luke says they were on cloud nine! They went back to the scriptures and found all the good news prophesied in the earlier books of the Bible and were filled with joy.