You can always write to your author in the traditional wise, via their publisher, who should forward such missives to where they need to go; but these days it's even simpler than that, because a vast majority of us scribblers have blogs, or websites, or other ways in which an email can be obtained - and you don't even need an envelope and a postage stamp. And you have no IDEA how wonderful it is to find a nice note tucked away in your inbox, telling you that your words reached somebody, that someone else loved your baby, that what you are doing has meaning and value and, in some manner, affects another human being in measurable ways.
In fact, I am going to go out and pen e-missives to half a dozen authors whose email or blogs I have access to. You don't need to have a special day for this - making another human being feel warm and fuzzy and good about themselves is a good deed at any time. Go on, do likewise. Make the day of some isolated word-wright, sitting there by candlelight and dreaming up new worlds for y'all to go journeying in. Go on. Do it. Do it now.
Tell an author you care.