Science, Tech, Math › Math Calculate the Exact Number of Days Calculate the Exact Day of the Week Share Flipboard Email Print It's a simple matter of math to use a loan or known date (like your birthday) to find the exact day of the week in the future for an event. Jeffrey Coolidge, Getty Images Math Math Tutorials Geometry Arithmetic Pre Algebra & Algebra Statistics Exponential Decay Functions Worksheets By Grade Resources View More By Deb Russell Math Expert Deb Russell is a school principal and teacher with over 25 years of experience teaching mathematics at all levels. our editorial process Deb Russell Updated January 07, 2018 An interest period will involve two dates. The date the loan is given and the end date. You will need to find out from the loan institution if they count the day the loan is due or the day before. This can vary. In order to determine the exact number of days, you will first need to know the number of days in each month. January - 31February - 28*March - 31April - 30May - 31June - 30July - 31August - 31September - 30October - 31November - 30December 31 You can remember the number of days in a month by memorizing the days of the months nursery rhyme: "Thirty days hath September,April, June, and November,All the rest have thirty-one,Excepting February alone,Which has but twenty-eight days clearAnd twenty-nine in each leap year. February and Leap Year We cannot forget about Leap Year and the changes it will present for the number of days in February. Leap years are divisible by 4 which is why 2004 was a leap year. The next leap year is in 2008. An extra day is added to February when February falls on a leap year. Leap years also cannot fall on a centennial year unless the number is divisible by 400 which is why the year 2000 was a leap year. Let's try an example: Find the number of days between Dec. 30 and July 1 (not a leap year). December = 2 days (Dec. 30 and 31), January = 31, February = 28, March = 31, April = 30, May = 31, June = 30 and July 1 we don't count. This gives us a total of 183 days. Which Day of the Year Was It? You can also find out the exact day that a specific date falls on. Let's say you wanted to know what day of the week a man walked on the moon for the first time. You know that it was July 20, 1969, but you don't know which day of the week it falls on. Follow these steps to determine the day: Calculate the number of days in the year from Jan. 1 to July 20 based on the number of days per month above. You will come up with 201 days. Subtract 1 from the year (1969 - 1 = 1968) then divide by 4 (omit the remainder). You will come up with 492. Now, add 1969 (original year), 201 (days prior to the event -July 20, 1969) and 492 to come ups with the sum of 2662. Now, subtract 2: 2662 - 2 = 2660. Now, divide 2660 by 7 to determine the day of the week, the remainder = the day. Sunday = 0, Monday = 1, Tuesday = 2, Wednesday = 3, Thursday = 4, Friday = 5, Saturday = 6. 2660 divided by 7 = 380 with a remainder of 0 therefore July 20, 1969 was a Sunday. Using this method you can find out which day of the week you were born on! Edited by Anne Marie Helmenstine, Ph.D.