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Compute the Day of any Date in Your Head

Calculate whether any date in history was a Friday.

Ever wondered what day your friend’s birthday occurred, or whether the next July 4th will be a Thursday? You can calculate it using simple math, which you’ll learn here!

This simple method uses different codes that we add up to get our result. We’ll need a month code, a century code, and a day code.

Calculate It!

As we explain the process, we’ll use December 16, 2482 as an example.

  1. Take the last two digits of the year. In our example, this is 8282.
  2. Divide by 4, and drop any remainder. For us, 82÷4=20\lfloor 82 \div 4 \rfloor = 20.
  3. Add the date of the month. We’ll do 20+16=3620 + 16 = 36.
  4. Add the month’s value from the month codes table. In our example, this is 66, so we do 36+6=4236 + 6 = 42.
  5. Add the century code from the century codes table. In our example, this is 66, so we do 42+6=4842 + 6 = 48.
  6. Add the last two digits of the year. In our example, this was 8282, so we do 48+82=13048 + 82 = 130.
  7. Divide by 7 and take the remainder. In our example, this is 130mod7=4130 \bmod 7 = 4.
  8. Find your day in the day codes table. In our example, this is Wednesday. This is your final result!

Month Codes

MonthValueMonthValue
January1*July0
February4*August3
March4September6
April0October1
May2November4
June5December6

* For leap years, use 2 for January and 5 for February.

Century Codes

CenturyValue
1700s, 2100s, 2500s, …4
1800s, 2200s, 2600s, …2
1900s, 2300s, 2700s, …0
2000s, 2400s, 2800s, …6

Day Codes

DayValue
Saturday0
Sunday1
Monday2
Tuesday3
Wednesday4
Thursday5
Friday6