The book "Our calendar. The Julian calendar and its errors. As corrected according to the Gregorian. Rules for finding the day letter and day of the week of any event from the time of Julius Caesar 46 BC to the fourth millennium of our Lord: a new and easy method " written by author George Nichols Packer. This book examines the history of the development of calendars, starting with the Julian calendar and ending with the Gregorian calendar. The author explains in detail the errors inherent in the Julian calendar and the methods of correcting them using the Gregorian calendar. In addition, the book provides rules for determining the day letter and day of the week of any historical event from the time of Julius Caesar to the distant future years. The author presents a new, easy method of solving these problems, which makes complex calendrical calculations more understandable and accessible for the reader of any level of training. If you are interested in the history of calendars, mathematics, and want to better understand how to define and compare time intervals according to different calendars, this book will be a useful source of information for you.