Initium aestatis in Latin literally means “the beginning of summer” and it was not a holiday for the Romans – confirmation is the lack of such information in both Fasti Antiates Maiores, the official Roman calendar of public holidays, and in Ovid’s “Fasti”.
The beginning of calendar summer (initium aestatis), is astronomically marked by the rise of the Pleiades constellation. According to Pliny the Elder1 it falls on the 48th day after the vernal equinox. The one in the Julian calendar is set 8 days before the April Kalends- that is March 25.
It would seem, therefore, that the Romans used May 12 as the first day of summer. In fact, however, different sources give different dates here. For Varro and Columella, the beginning of summer falls on May 9. For Ovid – May 13.
One of the reasons for this confusion may be intercalation errors (counting leap years) committed in 42-9 BCE. As a result, the calendar began to lag behind, and this delay in 13 BCE was two days. It was this year in the Field of Mars that the construction of a great sundial called Augusti Solarium began. It also served as a calendar, and the beginning of summer is marked on it on May 7. Pliny the Elder in the 1st century CE states that this calendar has gone awry and is wondering about the causes (the reason was the delay-compensating intercalation adjustment carried out in 8 BCE – 4 CE).
The Romans assumed the beginning of the astronomical summer – as we do so far – as the day of the summer solstice (solstitium). In the Julian calendar, this date falls on the 8th day before the July Kalends (June 24).
Pliny the Elder, Natural history, XVIII.68
east of the Pleiades on the 48th day after the vernal equinox: Plin. Nat. XVIII 222
Vernal Equinox March 25: Plin. Nat. XVIII 246
beginning of summer by Varro: Var. RR I 28
beginning of summer according to Columella: Colum. November 34
beginning of summer according to Ovid: Ov. Fast. V 599-602
beginning of summer according to Augusti Solarium: Brill's New Pauly, slogan Seasons
deregulation of Solarium Augusti and hypothesizing about the causes: Plin. Nat. XXXVI 72-73
beginning of the astronomical summer June 24: Plin. Nat. XVIII 264.
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