In MLA format, when citing a website, you typically start with the name of the website. Therefore, the correct information to list first for this source is the name of the website: A Day for Trees.
Here is how the citation would begin:
A Day for Trees. U.S. Department of Agriculture, 10 Jan. 2014. Accessed 18 Apr. 2014. www.arborday2014.org.
Make sure to format it correctly according to MLA guidelines with appropriate punctuation and italics as needed.