Arbor Day

I never thought I'd be one of 'those' people, who bemoan the death of the English Language. Or who complain about declining standards of literacy or dumbing down of society, after all I live in Australia. But it turns out I am.
What the hell happened to Arbor Day, and why the hell do we have Tree Day.
As a kiddie I had lots of fun on Arbor Day planting trees. There are a couple of nice big Gum Trees on Ham Common which I can still identify as ones I planted. Now however (and for the last few years) Australia has tree day. Why?
Now I know it isn't dumbing down the English language, because Tree is English but Arbor isn't. Still it is (at the very least) aiming for the lowest common denominator. And I hate that!