Well, lots of things in life are completely arbitrary. Just do what works for you.

Because you're surrounded by people with emotions and feelings and one of those feelings is to be made feeling extra-extra-special and an anniversary day naturally slips into that.

Does that line of thought inevitably lead to nihilism? I mean, isn't life just as arbitrary?

Almost all of life is arbitrary and meaningless. Denying this is not particularly enlightened, and ultimately tends to get rejected by the intellectually curious. On the other hand, seeing absolutely no meaning results in wasted time. The least bad answer seems to be leading a balanced life, with a few elements that are very important, from which meaning is created.

so, to summarize, Yes but there's no point getting all upset about it.

Richard Dawkins has a quote along the lines of, "Why do we have to look at a beautiful planet and demand a reason behind its being here? Can't we appreciate it for what it is?"

So it is with life. Perhaps in the long run we'll all be dead and the universe will cave in, but in the meantime the things we do and say matter, and there is beauty in the world to appreciate. So I went to the boardwalk with my father and brother and we all had a terrific time, and having a good time was reason in itself to do it.

Once you give up the religious/gamers' view that everything you do has a score, you can appreciate things for what they are rather than for any ultimate result.

Life boils down to aesthetic choices, best as I can tell.

At the risk of sounding like I don't have a sense of humor- it's not really arbitrary (at least not until you dig way deeper). Celebrating at a time when the earth has completed a revolution allows people to remember exactly when to celebrate- kind of an old-school time-management trick, so to speak. If we didn't have calendars, we could still have rocks sticking up with a line engraved somewhere on the ground to give us that same reminder- almost exactly once every year.

It's a convenience. ;)

Classic. Absolutely classic.

Because we get a lot more done that way than if we packed all those celebrations into one earth's rotation about it's own axis.

We could pack them into one rotation of the sun around the galactic center and get a lot more done, but then lots of lifes would be without a single celebration.

Something can be arbitrary and still worthwhile.