"月的第十二日" can be translated as "the twelfth day of the month" in English.
If you're referring to a specific month (though the original Chinese phrase doesn't specify), you could say something like "the twelfth day of January" (一月第十二日) or "the twelfth day of February" (二月第十二日), etc. But in the general context of just translating "月的第十二日", "the twelfth day of the month" is the appropriate phrase.