Unlike the simple way we number our calendar daily, the Romans numbered theirs in groups or blocks of days that are a little more complicated for our simple modern system.
There are three blocks of days in the month in the Roman calendar.
The first block is the Nones, which ends on the 12th or 13th day of most months.
The second block is the Ides, which ends on the 22nd day of most months.
The third block is the Kalends, which ends on the Ist day of most months.