Thirty Days Hath September

This is a mnemonic verse used to remember the number of days in each month of the Julian and Gregorian calendars, with origins traced back to medieval Europe and specific connections to the 16th-century calendar reforms. An early precursor appeared in a 13th-century French poem, and an early English version was recorded in a 1425 manuscript that contained information about saints' days, with the verse appearing in print for the first time in 1562. A variant was referenced by chronicler William Harrison in 1577, often starting with "Thirty days hath November" before evolving into the more familiar form, and the use of the word "hath" instead of the modern "has" points to its origins dating back to at least the 16th century. The irregularity of month lengths, which necessitated such a mnemonic, stems from the ancient Roman calendar system, and the verse's continued use was particularly relevant given Pope Gregory XIII's institution of the Gregorian calendar in 1582 to correct the Julian calendar's slight inaccuracy. The rhyme continued to be a valuable tool for navigating these calendar systems and their fixed, though irregular, month lengths, with similar mnemonics also existing in other European languages, reflecting a broader tradition of using verse to aid memory in the pre-modern era.