Which is the First Day of the Week:
Sunday or Monday?

Sunday was traditionally regarded as the first day of the week by both Christians and Jews. Following Jewish tradition, the Bible is quite explicit that God rested on the seventh day of Creation, which formed the basis for the Sabbath, the day of rest.

Even when Constantine made Sunday the Christian day of rest, thus giving it the traditional functions of the Jewish Sabbath, there was no question that Sunday remained the first day of the week. The week introduced by Constantine treated Sunday as Kyriaki 'of the Lord', followed by numbered days from Monday to Thursday ('second' to 'fifth') and specific names for Friday and Saturday. Saturday was Savvato, the Sabbath.

(As to whether Constantine was swayed by his previous worship of the Sun God, or whether he simply ratified a practice that already existed among practising Christians is a point of heated debate among certain modern Christians. It does seem true, however, that Constantine was motivated by a spirit of compromise rather than doctrinal purity. A lot of information about this can be found on the Web).

Notwithstanding Constantine, the custom of treating Monday as the first day of the week may have appeared quite early. This is explained by Michael Falk as follows:

We may note that in early Christian coinages Saturday is denoted as day seven, following the Bible. In later coinages, however, it is denoted as day six. This change appears to have been brought about by a natural tendency to consider the celebrated special day of the weekly cycle as ending the cycle rather than beginning it. So, after about the tenth century for most Christians Sunday replaced Saturday as day seven. An analogous change occurred in the Islamic world where Friday began as day six but later became counted as day seven.

Thus, when saints Cyril and Methodius brought Christianity to the Slavs, they took with them the Greek tradition of numbering the days but numbered them from Monday instead of Sunday.

As a result, Slavic languages now treat Monday as the first day, Tuesday as the second, etc. Saturday and Sunday are the only days that are named rather than numbered. Saturday is the Sabbath and Sunday in Russian is the word for "Resurrection". (The foregoing information on Greek and Slavic naming due to John Wilson, personal communication).

In Hungarian, which is a non-Slavic language in a Roman Catholic country, Tuesday appears to come from the word for 'two' (but see note below). Lithuanian and Latvian, neither of which is Slavic, also name the days with numerals, with Monday coming first.

www.friesian.com takes a different view of the change to Monday as the first day of the week:

The convention, becoming more common, to start calendar weeks on Monday, is a result of the Western European names, especially the German ones, which do not call Saturday the Sabbath — or do not do so anymore in a recognizable way. Since Christians, especially Protestants, think of Sunday as the "Sabbath," the tendency is to number it as the 7th, rather than as the 1st, day. Familiarity with Greek or Arabic, or several Romance languages, however, would inform one that Saturday remained the Sabbath, as in Hebrew, even for Christians and Muslims.

In more modern times, industrial society has done much to destroy the traditional concept of Sunday as the first day. According to Webster's Ninth New Collegiate Dictionary (1983), the term 'weekend', first recorded in 1878, refers to 'the period between the close of one working or business or school week and the beginning of the next'. This concept firmly places Sunday at the end of the week.

Possibly because of this, the International Standards Organisation has decided that Monday should be regarded as the first day of the week. Calendars in many European countries, in particular, now follow the ISO decision by starting the week on Monday. Airline timetables also number the days from Monday as 1, Tuesday as 2, Wednesday as 3, etc.

Information on the web is rather fragmentary, but references can be found at On the Name of the Weekly Day of Rest by Michael Falk, Is Sunday or Monday the first day of the week?, the Catholic Encyclopedia (Sunday, the Sabbath, and Liturgical Week), the Days of the Week page, Claus Tøndering's Calendar FAQ, International Units: The Week, and Wikipedia.

Re Hungarian: The following note was received from Gyorgy Kereszti:

In the Hungarian language, the name of Tuesday does not really come from the number two.  Tuesday = kedd and two is either két or kettö.  The name of Tuesday starts with “k”, but there is no definite relation between the name and the number two. 

However, Monday literally means "the head (or start) of the week".  Monday = hétfö, where hét = week and fö = head (or beginning of something).  Therefore, Monday in Hungarian is the first day of the week.

Sunday, in Hungarian, means the market-day: Sunday = vasárnap, where vásár = market and nap = day (the first vowel is modified). Market-day is generally considered to be the last day of the week.

All other days have widely different names (as opposed to languages with a prefix followed by “day”, like Monday or Montag (German)) and none includes any resemblance to numbers or sequencing.

Thanks Gyorgy!