My birthday was a few days ago. Time for thinking again. But I'll spare everyone my thoughts on my own life, so common as we get older (Actually, I believe I started at 18). No, today I'm thinking about birthdays in general. And anniversaries too, since birthdays are just special anniversaries.
So what is special about adding another mental notch for the 'turning of a year'? (that's what anniversary literally means, so strictly speaking, a monthly anniversary is a misnomer). Well, to start with the obvious, the Earth is in the same spot, with respect to the sun, that it used to be on the day of the original event. So what? Well, if you are the type of person that believes the position of the stars matters somehow, that is a big deal, so that's what our ancestors marked. If not, you may consider decimal birthdays to be symbolically more meaningful, really.
You reach 1,000 days before you turn 3, and 10,000 when you're 27 years old. Most of us will not live to see 30,000 sunrises, and even the vast majority of ultracentenarians won't make it to 40,000. It seems like such a large supply of days, and yet it's limited. Calculating your age in days has a strange effect: you can't remember living, say, 9,735 days; actually, you can't even fathom that, as our mind handles poorly even numbers in the thousands. Not only can't we remember living each and every day, or even a detail from each day; the appalling reality is that we probably remember vague details of only 1 in 100 days, and can easily recall a full good mental picture of only a few dozens days that were significant to us. And even then, recent days are represented far more often than older ones. Try it. Sit down in a quiet place and take notes, in writing, of the days you remember in detail, it's an enlightening exercise. Yearly anniversaries seem to make a lot more sense in light of the information we can store; not to mention we get to celebrate more often with yearly, rather than decimal anniversaries (1,000 days).
But exactly why do we celebrate? Is it wise or justifiable to celebrate getting older? That very much depends on your perspective. When we're very young we can't wait to get older, primarily because reaching certain specific ages removes societal restrictions on our behavior, and unlocks entire new life experiences, much like reaching a new level in a videogame: at age w you can wear big-boy/girl underwear, at age x you can stay home alone, at age y you can date, at age z you can drive, etc. Between reaching the driving/voting, or in the US the drinking age, and retirement age, there are no longer any magic numbers to look forward to, and we may lose the taste for celebration. In fact, our professional careers offer many similar goals that are tied, however losely, to age, but after the internet age produced a number of 20-something billionaire CEOs, the connection is weaker than ever. Most of us probably do gain wisdom as we age, but you can't get a numerical feedback for that, and it doesn't sound like that much fun anyway. And after retirement, we start contemplating times ahead when we may progressively receive more and more restrictions: we may lose our driver's license, control of our finances, and even of our bladder. So what is there to celebrate?
Actually, a lot. While upcoming goals are certainly a good reason to celebrate, the true, overwhelmingly important reason to celebrate is that we are still here, we made it through another year against all odds. It's a victory, and victory deserves celebration. What am I talking about, most people live to be old, anyways? Yes, they do, but there's nothing obvious about it.
Both physics and biology paint grim pictures of life. Thermodynamically speaking, as Erwing Schroedinger observed, life should not exist. In fact, we die. Eventually. If you know the concept, consider that the change in Gibbs free energy (delta G) produced by our death is hugely negative, so nature strongly favors that outcome. It's only a matter of time, before nature takes its natural course.
Biologically speaking, if you haven't been eaten by a predator today, whether a large feline or a microscopic parasite, you should count yourself successful: most of your fellow organisms on this planet put a huge effort into avoiding just that, with uncertain outcomes. And even us humans, so successful as to be generally delivered from such concerns, do sometimes turn into food for predators, typically of the invisibly small kind.
These two scary monsters, thermodynamics and predators, are responsible for a statistical paradox: the older you are, the older you are likely to get. For example, in the US in 2005, if you were 5, your odds of reaching 90 were only 22.5%. But if you were 60 already, odds would improve to 25.4%, and had you been 75, they'd jump to 33.4%! You cleared many, many life-threatening obstacles in the past years of of your life, including the latest one. And you should celebrate that victory.
Similar forces threaten anything else we celebrate with an anniversary: romantic relationships fail or fade for myriads of reasons all the time; once proud and successful businesses sink or close doors; even world-spanning empires disintegrate. All these frail things seem to have an especially high mortality early on, and that was the case for human life too, until very recently. No wonder that endurance in the face of Time has always been a cause of celebration, still is, and always will be, as long as Time brings change and loss.
One question remains nagging us, though. Relationships, institutions, and even we ourselves change into unrecognizable entities over time, and we scarcely remember a handful of glimpses of what we have been. What, then, do we have in common with the person that we call ourself and lived 10 or 20,000 days ago, that we should celebrate such continuity?