I'd never heard of this book (written in 1938) until recently, when I started running across references to it in a lot of disparate places. It's fascinating, and I wonder if play-theory is going to make a comeback as a major school of literary and social criticism. "Play," after all, is one of the few values that has remained intact across both New Criticism and postmodernism (although I've always found that postmodernism plays lip service to play, favoring narrow identity politics). Huizinga's thesis is that play and play-forms both predate and comprise human culture. This is a more complicated assertion than it may first appear: many aspects of culture that we consider play actually do not meet Huizinga's formal definition (notably, professional sports) whereas traces of actual play, properly rooted in serious and extra-rational ritual, is often hidden.
I'd like to reread this book someday when I have a more firm grounding in Classics and 18th century philosophy--probably not until Matrix downloading technology has been perfected--because I feel as if I absorbed only a small fraction of what is offered here. But I am glad that I'd recently read Jane McGonigal's Reality Is Broken: Why Games Make Us Better and How They Can Change the World so to better draw connections between Huizinga's theories and the changing conception in contemporary life of what constitutes play. A major problem I had with the McGonigal book--the fact that she ignores the economic dimension of how video games are consumed--finds complementary criticism here where Huizinga rigorously defines the circumstances under which "play" can occur in a number of contexts.
His approach is highly philological, but even without knowledge of ancient Greek (which, don't get me wrong, would have helped), I still found very interesting Huizinga's instances of how the play-element is embedded in language itself, from Greek to contemporary Dutch (his own language) and English (whose word "fun" Huizinga claims has no perfect analogue in any living language). I'm going to be looking for post-war criticism of Huizinga's ideas; play-theory (not to be confused with game theory, also interesting, but better aligned with economy-centered ideologies) seems to be one of the most constructive lines of cultural inquiry I've been acquainted with in a while.