The seemingly simple Gregorian calendar we use today hides many secrets. Where did the names of the months come from and why do they have different periods? Why does the second month contain an extra day every four years and why is this year inexplicably called "leap"? When do centuries and millennia begin? This book is not just an encyclopedia. In addition to alphabetical articles, it contains more than six, sometimes extensive, essays on a wide variety of calendar topics, which are presented rather briefly in other books.