Final answer:
The shift from Saturday to Sunday for observing the Third Commandment occurred due to the emergence of Christianity, the association of Sunday with Jesus Christ's resurrection, and the adoption of Sunday as a day of rest by Roman Emperor Constantine in 321 CE.
Step-by-step explanation:
The observation of the Third Commandment, which calls for keeping the Sabbath day holy, shifted from Saturday to Sunday primarily due to the rise of Christianity, which associated Sunday with the resurrection of Jesus Christ. In the Jewish tradition, the Sabbath, or Shabbat, is observed from Friday evening to Saturday evening, marking a day of rest and worship. As Christianity emerged and separated from Judaism, early Christians began to mark Sunday as their day of rest to honor the day Jesus was believed to have risen from the dead.
This change was further solidified by the Roman Emperor Constantine, who in 321 CE decreed Sunday as a day of rest for the Roman Empire. This political decision helped to formally establish Sunday observance for Christians. Over time, this tradition became widespread among Christian communities, and Sunday was adopted as the official day of rest and worship, aligning with the Christian interpretation of the Commandment.
The observation of Sunday as the Christian Sabbath represents not only a theological shift but also a cultural and legal adaptation as seen in various laws regarding Sunday rest and activities throughout history and across different societies.