Year Anniversaries....What do they mean to you?

What does it mean when we say, "it will be a year, since...."? What does that year anniversary mean? It could be a first year of marriage, a first year of someone passing away, a first birthday? What ever it is, we seem to find that the first year is a big milestone, why is that? We don't just do that with one year, but it could be with days, months and years....yet that first year has a meaning behind it, like a secret code. What does that code me? It's one of those things in life that I don't know if there is a specific answer too, yet for me that first year let's me know that I am still here and that whatever the situation, I am still here to go through another year.