Q. In the sentence “It happened on the twenty-fourth of July,” should the date be spelled out or a numeral? CMOS only addresses the treatment of ordinals when the month is not mentioned.
A. We get this question a lot. CMOS now specifies that ordinals for days mentioned without the month need not be spelled out. So you can refer to “the 24th of July” for July 24 (but not “July 24th”). See paragraph 9.33 in the 18th edition.