“Happy anniversary!”

A symphony of applause, laughter and kazoo erupts as the aging couple enters the gymnasium. Party hats are promptly fastened to their crania as they are escorted to their seats at the head table by two of their young grandchildren. They take their chairs below a sparkling banner which reads, “Fifty and Counting!” Joy and gratitude are written on every crease and wrinkle as smiles engulf their faces. Waves of emotion crash over them as they recognize one familiar face after another, some they’ve not seen in decades. The shock is amplified when it becomes clear that many guests have migrated great distances to show their support. This is truly a magical evening for the aging couple, but why tonight? Why is this anniversary so much more important than the last?

Anniversaries are an important part of our lives. They are achievements, medals of accomplishment, which we wear with pride, reminding us of our past, passions and commitments. Anniversaries can mark anything from births and weddings to freedom and tragedy, but why are some anniversaries more significant than others? Why is the 50th year more meaningful or important than the 49th? This is because anniversaries are a mixture of two concepts: commemorative days and significant numbers.

Marking days on the calendar in honor of special events, both personal and public, has been a popular method of commemoration throughout history. It’s obvious why you would want to celebrate a birthday or wedding day, but the reason for the emphasis on certain years is more illusive. The cause, to put it plainly, is that we consider some numbers to be of more importance. Let’s take a look at the three importance factors which make up all significant numbers and attempt to unravel what makes them dearly beloved.

Two: Though not especially significant for anniversaries, the number two plays an important roll in calculating number importance, which we will discuss later. Although two does not add to a number’s significance, it is a vital component in the production and division of even numbers, which are much more well-liked than odd ones.

Five: By far the most significant odd number, five is a commonly found in years of commemoration. However, the number five, though compelling, borrows much of its importance from another number. This is the second most valuable factor.

Ten: The most valuable factor and the basis of all number significance and commemoration, ten is the backbone of all monumental anniversaries. Every important year is created using the numbers two, five and ten. The more fives and tens used to create a number, the more significant it becomes. Here’s an example of a few important anniversaries:

  • 20th: A product of ten and two, arguably more significant than the 10th, but not as significant as the 25th.
  • 25th: A product of ten and five divided by two. Notice how this number incorporates more importance factors, which makes it more significant.
  • 30th: A product of ten and three, this anniversary is less significant than the 25th because it has a factor of three, which has no importance.
  • 50th: A product of five and ten, this anniversary is more significant than the 25th. This is because it is a product of five and ten but is not divided by two.

Getting the picture? Let’s put it into a formula:

  • Importance = (TenCount * 3) + (FiveCount * 2) – DividedByTwo
  • 20 = 10 * 2 : Importance(20) = (1 * 3)  =  3
  • 25 = 10 * 5 / 2 : Importance(25) = (1 * 3) + (1 * 2) – 1 = 4
  • 500 = 10 * 10 * 5 : Importance(500) = (2 * 3) + (1 * 2) = 8
  • 500 = 10 * 10 * 10 / 2 : Importance(500) = (3 * 3) – 1 = 8

Now that we understand how to calculate which anniversaries are the most significant, let’s look at some possible explanations for why two, five and ten are considered important to us.

First, the anatomical explanation. We have five fingers on each hand, ten in total, so we are used to counting in fives and tens. Our bodies also usually have two feet, two lungs and two eyes, among other useful things. Because of the natural occurrence of these numbers in our daily lives, we could be drawn to them. But, if this is the case, why would ten be more important than two or five?

One argument is that these numbers are easier to deal with mathematically, therefore we like them. A dime is worth ten cents not because of our love for the number ten, but because having a coin worth ten cents is a practical way to exchange change. This is a persuasive argument, but would only explain areas which require calculation, such as physical measurement and currency.

Another theory is that all of our number affinity is caused by the decimal numeral system. As children, we all learned to count to ten and, from an early age, began to recognize its significance. When we count up from one, everything is straightforward until we reach nine, then we must cross the threshold and carry over, restarting at one again. Perhaps this process burrows itself deep into our minds, inscribing the importance of the number ten onto the tablet of our subconscious.

So when you celebrate an anniversary, you are celebrating the cause for the commemoration as much as you are celebrating a significant number.