I feel very strongly about copyright. Put simply, what you write is yours regardless of what anyone else thinks unless you have specifically given away your copyright. If you write a letter to a friend, that's copyright. If you write a shopping list, that's copyright too. If you write an article and sell it to a newspaper who put it up on the web, that's also copyright even though it's freely available to read and print off.
Jane's declaration of Copyright Day came about because someone at a magazine in the US didn't realise there was a difference between being publicly available, and in the public domain. Public domain is when an author has specifically chosen to give away their copyright. The magazine editor was copying and using previously published articles without consulting (or paying) the authors. What happened next is on Jane's blog and so I'm not going to cover it here.
It is important to maintain copyright because without it, no author will ever get paid. Why would anyone pay when they could help themselves to the material for free? I feel particularly narked when people who are on salaries complain about paying for material, saying that writers should feel grateful that their work is being read. (This was one of the magazine editor's excuses for why they weren't paying writers.)
Well, hello? Plumbers won't work for the thrill of fixing your cistern, and the garage doesn't give petrol away for the hell of it. Writers have bills to pay too, just like everybody else on the planet. Copyright maintains writers. Support it.
If you want to know more about copyright, go to How Publishing Really Works. Nicola Morgan has also done a good blog post explaining the ins and outs of the law.