Theoni Pappas seems to have a mixed reputation (at least, if we go by Amazon reviews). Some people like her art deco approach to popularizing math and like the fact that it is aimed toward kids, parents, and self-learner adults who only have a vague recollection of high school math. Others diss her books as being too simplistic, too easy, too derivative, etc.
Whichever side of that argument you fall into, I think any reasonable person that is interested in math should agree (after perusing *Math-A-Day*) that this is clearly her best book and a good popular math book by any standard.
As others have pointed out, this book is organized as mathematical vignettes for 366 days of the year (which includes leap years). Each of these math capsules includes a brainteaser-type question that requires at most high school level math knowledge, some math trivia, and a math-related quote. From what I can tell skimming through it, the problems seem to be a nice mix of relatively accessible questions that aren't too hard all the way up to fairly challenging problems requiring a lot of sophistication (although, like I said, not much knowledge).
The problems and their solutions (included in the end of the book), as well as the math trivia, should be of interest to all ages and of particular interest to those with an interest in learning as much about math as possible. This is a great book and is definitely not for juvenile primates or for drunken neighbors as suggested elsewhere. Learning about interesting mathematical trivia -- like Mayan mathematics -- should be of great interest to any genuine math lover and other clever, intelligent people.
In short, whether you like Pappas' other books or you'd like to lump them, THIS book, *Math-A-Day*, is definitely an interesting book and worthy of the attention of math lovers.