Sunday is a day of rest in most Western countries, part of 'the weekend'. In most Muslim countries, Sunday is a working day.
According to the traditional Christian calendar, Sunday is the first day of the week. According to the International Organization for Standardization ISO 8601 Sunday is the seventh and last day of the week.
No century in the Gregorian calendar starts on a Sunday, whether its first year is '00 or '01.
The English noun Sunday derived sometime before 1250 from sunedai, which itself developed from Old English(before 700 Sunnandæg literally meaning "sun's day", which is cognate to other Germanic languages, including Old Frisian sunnandei, Old Saxon sunnundag, Middle Dutch sonnendach (modern Dutch zondag), Old High German sunnun tag (modern German Sonntag), and Old Norse sunnudagr (Danish and Norwegian søndag, Icelandic sunnudagur and Swedish söndag). The Germanic term is a Germanic interpretation of Latin dies solis "day of the sun", which is a translation of the Ancient Greek heméra helíouIn.
In the Christian as well as in Islamic tradition, Sunday has been considered as the first day of the week. A number of languages express this position either by the name for the day or by the naming of the other
days position of the week count.
The current Greek name for Sunday, Kyriake, means "Lord's Day" coming from the word Kyrios, which is the Greek word for "Lord".
Christians from very early times have had differences of opinion on the question of whether Sabbath should be observed on a Saturday or a Sunday. The issue for Seventh Day Adventist, for whom Sabbath is unquestionably on Saturday, nor for Muslims whose day of assembly is on a Friday.
The ancient Romans traditionally used the eight-day nundinal cycle, a market week, but in the time of Augustus, the seven-day week also came into use. The two weeks were used side-by-side until at least the Calendar of 354 and probably later, despite the official adoption of Sunday as a day of rest by Constantine in AD 321.
Mithraism kept Sunday holy in honor of Mithras. On 7 March 321, Constantine I, Rome's first Christian Emperor decreed that Sunday would be observed as the Roman day of rest:
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.
More about Sunday click here
Some excerpts from Wikipedia