<P> The ancient Romans traditionally used the eight - day nundinal cycle, a market week, but in the time of Augustus in the 1st century AD, a seven - day week also came into use . </P> <P> Justin Martyr, in the mid 2nd century, mentions "memoirs of the apostles" as being read on "the day called that of the sun" (Sunday) alongside the "writings of the prophets ." </P> <P> On 7 March 321, Constantine I, Rome's first Christian Emperor (see Constantine I and Christianity), decreed that Sunday would be observed as the Roman day of rest: </P> <P> On the venerable Day of the Sun let the magistrates and people residing in cities rest, and let all workshops be closed . In the country, however, persons engaged in agriculture may freely and lawfully continue their pursuits; because it often happens that another day is not so suitable for grain - sowing or vine - planting; lest by neglecting the proper moment for such operations the bounty of heaven should be lost . </P>

Who declared sunday as holiday for the first time in the world