What is the roman letter wall in the HBO series Rome?

Wall calendars, in Latin called fasti were a common feature of Roman civic life. The name, fasti, comes from fastus, the dies fas, the day of (holy) judgement - i.e. the day on which the praetor would sit in judgement; opposed to the nefasti, the days in which this was not the case. Those days could instead be used for religious ceremonies such as sacrifices. The Capitals A-H you can see in sequence on the left hand side. These are the so-called nundinal letters, which denote the nine-day week starting at A, ending at H (counting the days inclusively, i.e. the ninth and last day is also the first.) Every ninth (eighth) day was market day, the day on which the farmers came to town - which is why any new law had to be hung out visibly for at least three consecutive market days, so everyone had a chance to see them.

Next to it, the count of days and the related month-days, the kalendae at the beginning of the month, the nonae at the fifth or seventh day and the idus at the 13th or 15th of the month (mnemotic device: the months MOMJul had 31 days, so the nones and ides were later). The days were counted as, f.e., "the third day before the kalends of July" (again counting inclusively) = 29th June; PR stands for pridie k/non/eid or "the day before the calends/nones/ides of X".

A letter denoting special days for the citizenship, like a C, which would stand for C(omitiales dies), a day for a comitium or people's assembly. F for fasti dies, N for nefasti dies, NP (ligated, probably for nefas feriarum publicarum or nefas piaculum) for feriae, which were public holidays, days on which no (legal) business, but religious ceremonies were conducted. Ides were all NP days for sacrifices to Iupiter.

Other information concerning the day in question. This could be the name of a god or goddess to denote the days of a temples dedication (the dies natales) or historic events, such as the day of the foundation of Rome (ROMA COND(ita)).