Step 1. Converting date to Julian Day Number (JDN) If the date is equal or after October 15, 1582 AD and the reference country has introduced the New Style (Gregorian) Calendar, you must use the Gregorian to JDN formula: JDN = (1461 × (Y + 4800 + (M − 14)/12))/4 + (367 × (M − 2 − 12 × ((M − 14)/12)))/12 − (3 × ((Y + 4900 + (M - 14)/12)/100))/4 + D – 32075 where D is the number of the day of the month, M is the number of the month and Y is the year. If the date is equal or before October 4, 1582 AD and the year is equal or after 4713 BC you must use the Julian to JDN formula. This is so because the calendar used in this period was the Old Style (Julian) Calendar. Note that the Old Style Calendar was not introduced at the same time in all countries. Therefore if the Old Style Calendar is still in use in the reference country the Julian to JDN formula must be used. The January 1, 4713 BC is the start of day numbering introduced by Scaliger in order to avail a uniform system for measuring the number of days passed between two certain dates (See also info article 6: Joseph Scaliger and the Julian Date Number). Important remark: The years BC are set as negative numbers with absolute value minus 1, e.g. 10 BC must be set as -9, 2 BC as -1 and 1 BC as 0. Do not forget that the year following 1 BC is 1 AD: JDN = 367 × Y − (7 × (Y + 5001 + (M − 9)/7))/4 + (275 × M)/9 + D + 1729777 Step 2. Finding day of week given the JDN The day of the week W (0 for Sunday, 1 for Monday etc) can be directly determined from the Julian Day Number (JDN) through the formula: W = mod (JDN + 1, 7) The day of week of any date of the past or the future is given in the online date converter of our site, where the reference country is also taken into account. An interesting example is the date of the October Revolution of the Bolsheviks, which happened on October 25, 1917. The reference country is Russia, where in that period the calendar used was still the Old Style one. The true date in the New Style Calendar was November 5, 1917. To find the day of week of this date we either use the Julian to JDN formula if we set the date in its commonly referred Old Style “October 25, 1917” or use the Gregorian to JDN formula if we set the true date “November 7, 1917”. The result in both cases is the required day of week: Wednesday (4). Another example is the date of the Greek Revolution against Turks, which happened on March 25, 1821. The reference country is Greece, where in that period the calendar used was still the Old Style one. The true date in the New Style Calendar was April 6, 1821. To find the day of week of this date we either use the Julian to JDN formula if we set the date in its commonly referred Old Style March 25, 1821 or use the Gregorian to JDN formula if we set the true date April 6, 1821. The result in both cases is the true day of week: Friday (6)