How Many Days Until Friday Calculator

This is modular arithmetic on a cycle of length 7. Weekdays form the residue class ring of integers modulo 7, so the interval to the next occurrence of a target weekday is (target minus current, plus 7) modulo 7. The extra 7 exists only to keep the intermediate value non-negative before the modulo, since JavaScript and C-family languages return a negative remainder for negative operands. The numbering convention used here is the JavaScript getDay convention, Sunday equals 0 through Saturday equals 6, so Friday is 5. From Monday the result is (5 minus 1 plus 7) mod 7, which equals 4. A result of 0 means the target weekday is today, and the calculator then reports the following occurrence as 7 days out. Over longer spans the counts are almost but not quite uniform. A common year of 365 days is 52 weeks plus 1 day, and a leap year is 52 weeks plus 2 days, so exactly one weekday occurs 53 times in a common year and two weekdays occur 53 times in a leap year; the rest occur 52 times. Averaged over the 400-year Gregorian cycle each weekday occurs 20,871 times in 400 years, about 52.1775 times per year. Two caveats apply to the results shown. All arithmetic here uses whole calendar days in the browser local time zone rather than elapsed hours, so the hours figure is simply days multiplied by 24 and ignores the time already elapsed today, and it does not adjust for daylight saving transitions, which make one day 23 or 25 hours long.

The seven-day week is unusual among calendar units because it has no astronomical basis at all; it does not divide the lunar month of about 29.5 days or the solar year evenly. Its ancestry is usually traced to Mesopotamia, where the lunar month was marked at roughly seven-day intervals, and to the Hebrew Sabbath cycle, which is an unbroken count independent of any other cycle. In parallel, Hellenistic astrology developed the planetary week, assigning the seven visible moving bodies (Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn) to successive days. That naming survives directly in the Romance languages and, through Germanic substitution of local deities, in English Tuesday, Wednesday, Thursday and Friday. The planetary week spread through the Roman world during the first two centuries CE, and Emperor Constantine gave it official standing in 321 CE by decreeing the day of the Sun a day of rest for city dwellers. Its continuity has proved resistant to reform. The Gregorian reform of 1582 skipped ten calendar dates while leaving the weekday sequence unbroken, so the cycle ran without interruption across that transition. Two attempts to break it failed: the French Republican calendar of 1793 imposed a ten-day decade and was abandoned by 1806, and the Soviet Union ran five-day and then six-day work cycles from 1929 before restoring the seven-day week in 1940. The Monday-first numbering that competes with the convention used here was standardised in ISO 8601, first published in 1988.