Determination of the day of the week ISO 8601. The day designated with 7 may also be counted as "0", by applying the arithmetic modulo 7, which calculates the remainder of a number after division by 7. Thus, the number 7 is treated as 0, 8 as 1, 9 as 2, 18 as 4 and so on. If Sunday is counted as day 1, then 7 days later (i.e. day 8) is also a Sunday, and day 18 is the same as day 4, which is a Wednesday since this falls three days after Sunday. The basic approach of nearly all of the methods to calculate the
Names of the days of the week seven-day week spread throughout the Roman Empire in Late Antiquity. By the 4th century, it was in wide use throughout the Empire, and it had also reached India and China. The Greek and Latin names are as follows: Except for modern Portuguese and Mirandese, the Romance languages preserved the Latin names, except for the names of Sunday, which was replaced by "[dies] Dominicus (Dominica)", i.e. "the Lord's Day" and of Saturday, which was named for the Sabbath. In Corsican, the Saturday is known either by Sabatu or De Sadorn. Early Old Irish adopted the names from Latin, but introduced separate