The Roman calendar counts thusly: the Kalends is the first of any month. The 7th or 9th of the month is the nones. (nines). The 13th or 15th of the month (depends on length) is the Ides.
Pridie means ‘the day before’
So pridie kalendae Octobri is Sept 30. Kalendae Octobri, or Kal Oct, is Oct 1. Oct 2 is viii pr non oct, that is, 8 days before (pr) the nones of October.
Hence Beware the Ides of March means Beware of March 15th.
This is why tax turn in day is April 15—because the US government just didn’t want to put up with the jokes…and they sure didn’t want to go with April 1.