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 is followed by a number indicating the day: "Monday" is literally "star day one", "Tuesday" is "star day two", etc. The exception is Sunday, where rì (日), meaning "sun" or "day", is used instead of a number. In Swahili, the day begins at sunrise, unlike in the Arabic and Hebrew calendars where the day starts at sunset (therefore an offset of twelve hours), and unlike in the Western world where the day starts at midnight (therefore an offset of six hours). Saturday is therefore the first day of the week, as it is the day that includes the first night