The Ides Can Be on the 15th
You may know that the Ides of March -- the day on which Julius Caesar was assassinated -- was the 15th of March, but that doesn't mean the Ides of a month was necessarily on the 15th.
The Roman calendar was originally based on the first three phases of the moon, with days counted, not according to a concept of week, but backwards from lunar phases. The new moon was the day of the Kalends, the moon's first quarter was the day of the Nones, and the Ides fell on the day of the full moon. The Kalends' section of the month was the longest, since it spanned two lunar phases, from the full to the new moon. To see it another way:
- Kalends = New Moon (no moon to be seen)
- Nones = 1st quarter moon
- Ides = Full Moon (whole moon visible in the night sky)
When the Romans fixed the length of the months, they also fixed the date of the Ides. In March, May, July and October, which were (most of the) months with 31 days, the Ides was on the 15th. On other months, it was the 13th. The number of days in the Ides period [from the Nones to the Ides] remained the same, eight days, while the Nones period [from the Kalends to the Nones] might have four or six and the Kalends' period [from the Ides to the start of the next month] had from 16-19 days.
The days from the Kalends to the Nones of March would have been written:
- ante diem VI Non. Mart.
- ante diem V Non. Mart.
- ante diem IV Non. Mart.
- ante diem III Non. Mart.
- pr. Non. Mart.
- ante diem VIII Id. Mart.
- ante diem VII Id. Mart.
- ante diem VI Id. Mart.
- ante diem V Id. Mart.
- ante diem IV Id. Mart.
- ante diem III Id. Mart.
- pr. Id. Mart.
The day before the Nones, Ides, or Kalends was called Pridie.
Kalends (Kal) fell on the first day of the month.
Nones (Non) was the 7th of 31 day months March, May, July, and October, and the 5th of other months.
Ides (Id) fell on the 15th of 31 day months March, May, July, and October, and on the 13th on other months.
Ides, Nones on the Julian Calendar
If you find this view confusing, try Julian Dates, which is another table showing the dates of the Julian calendar, but in a different format.