I recently received a disturbing return asking me to remove an apostrophe from a word that was plainly a possessive and not a plural according to the context of the verse in question.  I thought I was being rather creative taking a staid phrase "Earth Day" and turning it into "Earth's Day" since it was meant as a birthday card offering for someone on that day.
 
Both days are special; one celebrates the Mother that keeps us all and the other a special day in one of her children's lives.  Why then must I make Earth's Day, plainly a possessive in this case, into a plural when it isn't one?
 
An immediate reply would be greatly appreciated, as April is around the corner and I'd like the card posted by then.

Here is the verse in question: the Product ID is 766160
 
"Dewy diadems decorate these greens The ones that only nature makes Vanishing upon the rising heat As day proceeds to night; May your special day be one of joy And your celebration bring you more Remember nature's bounty to explore As her gift to you. Happy Birthday on Earth's Day" (C) 2011 by P.K. Taylor
 
As anyone can see, I wished someone a happy day on Mother Earth's special day, thus the use of the possessive in this instance.  It was sent back for re-evaluation some time ago and Earth Day fast approaches, so I'd like an answer stat.

Hi, Petra,
 
The card in question has been approved as-is. Apologies for the mistake. Thank you for your patience.
 
Regards,
Reviewer 443

You're welcome, Petra. I wasn't the original reviewer but we're always happy to take a second look at a card. In the future, you can avoid delays by responding to the e-mail we send regarding the card. We'll see your question in a more timely fashion. Thanks, again, for your patience!
 
Reviewer 443