The study focuses on the tract Largum sero written by the Benedictine monk Jan of Holešov. Four fifteenth-century manuscripts of this text exist; two are preserved in the Jagiellonian library in Cracow, one in the Library of the National museum in Prague and one in the Herzog August library in Wolfenbüttel. There is also an old print from Olomouc from the mid-eighteenth century. The author of the article describes how the tract reached Czech and foreign scholars, presents available biographical data about Jan of Holešov and analyses the content of the text based on the description of folk customs. Simultaneously, he offers references and quotations of late medieval primary sources of mostly Czech origin, whose authors comment on the ideas further developed by Jan of Holešov. Finally, comparing the manuscript material and the old print, he underlines their more significant differences and briefly evaluates the importance of this medieval source.