This book is so in-depth, I couldn't begin to imagine just how long it took Ginger Marks to research and write. In marketing, you may have come across advice to use holidays such as these to market your product. It is no different in book marketing.
It is a very useful reference to own if your book's theme can fit into a given public holiday. I wish I could make use of it! My books never seem to fit in with anything. The holidays are listed by month and day, which can make searching through it a bit laborious and time-consuming. I'm wondering if it would make things easier if the holidays could have been listed by genre. Would that have made it easier to look for a reference to your book's theme? I suppose you could look each month up as they come to see if it's relevant to you. Most of the holidays and dates are so obscure that, I expect, many people wouldn't be aware that they even exist.
Examples of non-traditional holidays you can find in this guide are Cuckoo Dancing Week in January, For Pete's Sake Day in February and Electric Razor Day in March. I'd be curious to know if anyone has written books that fitted into these! Maybe there is a different meaning to them and they are not all literal. The book also lists the usual, well-known holidays such as Christmas and Easter.
Overall, I think it's a book that could prove handy if you have some spare time or you want to write something incorporating a holiday listed. Then, there is no excuse for not finding at least some inspiration!