This book is an expanded and revised edition of my former Russian book "The Origin of Jewish Holidays and Christian Worship," published in Gomel in 1926. The first part, about Jewish holidays, reproduces with some, sometimes quite significant, additions and changes the text of the corresponding chapters of the Russian book; the second part, about Christian holidays, was written completely anew, and new works on the Christian Eucharist were used, depicting its origin and original character in a different form than it was previously imagined