From a dear friend, Eliana Rivero, who did the research for me:
Which is the First Day of the Week: Sunday or Monday?
Sunday is traditionally regarded as the first day of the week. Even when Constantine made Sunday the day of rest, thus giving it the traditional functions of the Jewish Sabbath or seventh day, 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 appeared quite early. When Saints Cyril and Methodius brought Christianity to the Slavs, they took on the Greek tradition of numbering the days, but being far removed from the Jewish origins, they numbered them from Monday instead of Sunday.
Thus, Slavic languages 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 is 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’. Lithuanian and Latvian, neither of which is Slavic, also name the days with numerals, with Monday coming first.
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 is to 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 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, Larry Freeman’s Calendar Origin Page, Claus Tøndering’s Calendar FAQ, International Units: The Week, and Wikipedia.