the name of the website
When creating an entry for a Works Cited page, the first piece of information should be the author's name. However, when the author is not available, you skip that information. Do not leave a placeholder. The next piece of information is the title of the source. Since this is a website, you'd need the title of the website: A Day for Trees.
The entire entry would be: A Day for Trees, U.S. Department of Agriculture, 10 Jan. 2014. <web address>. Accessed 18 April 2014.