The Ancient Roman Calendar
The Roman calendar is essentially the Julian calendar, but using the Romans' different way of expressing dates. They did not count forwards from the first day of the month, but instead counted downwards towards three marker days in each month. These days were the Kalends, the Nones and the Ides. The Kalends were on the first of the month. The Nones were on the 5th of the month for all months except March, May, July and October, when they were on the 7th. The Ides were always 8 days later than the Nones, i.e. on the 13th or 15th of the month. The intervening days were counted backwards, inclusively, to the next of these main days. So, for example, the 15th of March was the Ides of March, or Idus Martiae. The 14th March was Pridie Idus Martias, or the "day before the ides of March". The 13th was the 3rd day before the Ides, or ante diem III Idus Martias, showing the inclusive nature of Roman counting, i.e. three days before the 15th was the 13th inclusively (13 - 14 - 15).
This system was used for all the three main days of the month, and thus produced something that seems odd to us now. The dates following the Ides of each month refer to the Kalends of the following month, e.g. the 18th December was ante diem XV Kalendas Ianuarias, or "15 days before the Kalends of January", and the year number was incremented accordingly.
These dates were usually abbreviated, e.g.
Idus Martiae = Id. Mart.
Pridie Idus Martias = Prid. Id. Mart.
ante diem III Idus Martias = a.d. III Id. Mart.
ante diem XV Kalendas Ianuarias = A.D. XV Kal. Ian.
These abbreviations are used for the Roman date on the main calendar page.
Nundinal letters and market days
In the Roman calendar, the nundinal letter was like an equivalent of the modern days of the week, except with eight days instead of seven. Each day was lettered successively from A to H. Every eighth day (ninth day using the Roman inclusive counting system, hence the word nundinal) was a market day, which was a special day in Roman times, so wthhin a particular year the market days all had the same nundinal letter, and that letter was designated as the nunidanl letter for that year. Each date had the same nundinal letter from one year to the next, because the sequence was reset to 'A' at the beginning of the year. This meant that, applying the strict eight-day rule for market days, the letter for the market day changed from one year to the next. So, for instance, in 2007 the market day letter is 'H', which means that the last market day of the year would have been 26th December. The next market day would be 3rd January, so the market day letter for 2008 would be 'C'.
In leap years, the 6th day before the Kalends of March (a.d. VI Kal. Mart.) was effectivley doubled, hence why a leap year is often called a bissextile year. Effectively, the 24th of February in a leap year is inserted, thus pushing the remaining days of that month back by one day. The 25th, then, has the same nundinal letter as the 24th, and the remaining dates have their correct nundinal letters. So, for instance, Prid. Mart. always has nunindal letter 'C', even though in our modern calendar it is 28th February in a normal year, and 29th February in a leap year. To the Romans they were the same date. In the calendar, the second of these days is labelled bis, i.e. a.d. (bis) VI Kal. Mart.
The epoch for the year that has been used here is 753 B.C.E., which is the most favoured date for the foundation of the city of Rome, thus 2007 C.E. is the year 2760 A.U.C. (ab urbe condita - "since the foundation of the city").