Two Revisions to D-G’s Table of Julian Equivalents for Roman Dates 

for the Years 59 and 58 and for the Years 55 and 54

58 BC, not 59, was intercalary

D-G 3, 774 expressed uncertainly over whether intercalation took place in 59 or 58. Although they arbitrarily decided in favor of 59, two considerations make it far more probable that 58 was the intercalary year. The chief clue is furnished by the pattern of recurring “market days” (nundinae), which fell every 9th day (by inclusive reckoning), thereby dividing the year into 8-day units of time resembling our 7-day weeks. 

The argument goes as follows: if intercalation occurred in 59, instead of 58, then the day to which the consul M. Bibulus postponed the elections in 59, a.d. xv Kal. Nov. (Oct. 18) - Cic. Att. II, 20, 6 (= 40 SB) -  was a market day, which made it unlawful for a voting assembly to meet (A. Michels, The Calendar of the Roman Republic, Princeton 1967, 40-41).  

If there was no intercalation in 58, it is a certainty that a.d. xv Kal. Nov. 59 (Oct. 18) was a market day (and it is so designated by DG 3, 793)  because precisely 744 days,  a number divisible by 8 (93 nundinal intervals), separate that day from a.d. x Kal. Dec. (Nov. 21) 57, which Cicero (Att. IV, 3, 4 [= 75 SB].4) explicitly identifies as a market day [Those 744 days comprise 71 days in 59 (13 days in Oct. [19-31] + Nov. 1-29 + Dec. 1-29) + 355 days in 58 + 318 days in 57 (355 less 8 days in Nov. [22-29] and Dec. 1-29)]. So, given the high improbability that Bibulus called for an election on a date on which it was unlawful to conduct one, the logical assumption is that Oct. 18, 59 BC was not nundinal. This indeed is true if intercalation occurred in 58, not 59, since in that case the attested market day Nov. 21, 57 was separated from Oct. 18, 59 by 744 + 23 days, = 767, a number not divisible by 8 (See A. LINTOTT, Nundinae and the Chronology of the Late Roman Republic, CQ XVIII, 1968, 192 and R. KASTER (ed.), Cicero, Speech on behalf of Publius Sestius. Oxford 2006, 394-97, who adds supporting arguments drawn from the political activities of the tribune Clodius in the opening months of 58).  

And a further argument in favor of inserting the intercalary month in 58, instead of 59, is that the Kalends of Jan. 58 ceases to coincide with a market day, as it does under D-G’s scheme and is so marked. [I thank my friend John D. Morgan for pointing this out to me].  The Romans are said to have regarded it as inauspicious for a year to begin with a market day, and they tried to avoid that happenstance whenever possible (Macrob. Sat. I, 13, 17; D.CASS. XL, 47, 1-2 commenting on the year 52 BC; D.CASS. XLVIII, 33, 4, a single day intercalated in 41 to prevent a market day from coinciding with Kal. Jan. 40).

Torna al Calendario dell'anno 59

Torna al Calendario dell'anno 58

55 BC, not 54, was intercalary

An inscription published in AE 1992:177, which makes reference to the Kalends of an intercalary month in the second consulship of Pompey and Crassus (55 BC), settles the issue once and for all in favor of 55 over 54: 

Rufio / Vevei // Cn(aeo) Po(mpeio) M(arco) Li(cinio) II // sp(ectavit)

Kal(endis) Int(ercalaribus).

Torna al Calendario dell'anno 55

Torna al Calendario dell'anno 54