DAY/5.6 (in fact (D)DAYYYY/5.6) for 8-digits calculator "(D)D" is the date in the month (one or two digits). "A" is the month key value taken from the clock face (JAN & FEB are taken as months of the PREVIOUS year). 2 3 4(prev) 1 6(prev) 9 3 7 5 5 4 6 "YYYY" is the year (adjusted only for Jan and Feb by "-1"). After dividing (D)DAYYYY by 5.6, consider only the first two digits of the decimal part of the quotient and interpret the result as a DayOfWeek: .00+ Sun (in fact between .00 and .13) .14+ Mon .14 .27 .28+ Tue .28 .41 .42+ Wed .42 .55 .56+ Thu .56 .69 .70+ Fri .70 .83 .84+ Sat .84 .99 That's all !!! -------------- Example: 20 Jan 1975 1) For Jan use the key value A=4 and the previous year 1974. 2) Concatenate DD=20, A=4, and YYYY=1974, as 2041974. 3) Divide by 5.6: 2041974/5.6=364638.21 (only two decimal places). 4) Interpret .21 as Mon !!! (As Al Stanger says: "21 cents can buy only Mon, but not Tue !!!") -------------- An EAZY way for MEMORIZING the month key values "A": 1. As Al Stanger suggested, the months Mar(3), Jun(6), Sep(9), and Dec((1)2) are at their places on the clock face. Since only one digit is the key value, you have to take the last digit for Dec(2). As a mathematician, I would continue with arithmetic progressions: 2. Dec(2), Jan(2+2=4), and Feb(4+2=6) form a progression and you simply have to add 2 for the next month. Remember that Jan and Feb must be used with the previous year. 3. Another progression begins with Jul(5), Aug(5+2=7), Sep(7+2=9), Oct(9+2=(1)1), Nov((1)1+2=(1)3). Again you have to add 2 for the next month, but do not forget to take only the last digit for Oct(1) and Nov(3). (You can also start from Sep(9), which is at its place, and make two steps down(-2) and two steps up(+2) taking only the last digit !!!) 4. The only remaining months are Apr and May, but you can simply interchange their natural values: Apr(5) and May(4). (Sorry, I can treat this also as a progression, but here it is not with difference 2, but "minus 1" !!!) I hope everyone can memorize the key values and then the DAY/5.6 method can be used for Mar 1900 - Feb 2100 in the Gregorian (new) style. For other centuries one of the possibilities is to make the following adjustments for the month key values A: +3 for 18XX +6 for 17XX +4 for all years in the Julian (old) style Important note: If the result is bigger than 9, then subtract 7 and use the result as the one digit month key value. If the year is not 17xx, 18xx, 19xx, or 20xx in the Gregorian (new) style, then add/subtract 400, 800, etc. in order to have a modified year as above and work with it. For Julian (old) style no change of the year is needed. Another possibility is to use always the key values from the clock face presented above, but for other centuries to have different DayOfWeek tables corresponding to the decimal part of the result: | Julian | Gregorian | | | | | any | 20xx&19xx 18xx 17xx | | | | .00+ | Sat | Sun Mon Tue | .14+ | Sun | Mon Tue Wed | .28+ | Mon | Tue Wed Thu | .42+ | Tue | Wed Thu Fri | .56+ | Wed | Thu Fri Sat | .70+ | Thu | Fri Sat Sun | .84+ | Fri | Sat Sun Mon | As you see, there is one DayOfWeek difference between the columns!!!