Please note that Mommsen uses the AUC chronology (Ab Urbe Condita), i.e. from the founding of the City of Rome. You can use this reference table to have the B.C. dates
From: The History of Rome, by Theodor Mommsen
Translated with the sanction of the author by William Purdie Dickson
Spirit of the System
The whole system was pervaded by the utter regardless-ness characteristic of the power of capital. Slaves and cattle stood on the same level; a good watchdog, it is said in a Roman writer on agriculture, must not be on too friendly terms with his "fellow- slaves." The slave and the ox were fed properly so long as they could work, because it would not have been good economy to let them starve; and they were sold like a worn-out ploughshare when they became unable to work, because in like manner it would not have been good economy to retain them longer. In earlier times religious considerations had here also exercised an alleviating influence, and had released the slave and the plough-ox from labour on the days enjoined for festivals and for rest.(9)
9. Columella (ii. 12, 9) reckons to the year on an average 45 rainy days and holidays; with which accords the statement of Tertullian (De Idolol. 14), that the number of the heathen festival days did not come up to the fifty days of the Christian festal season from Easter to Whitsunday. To these fell to be added the time of rest in the middle of winter after the completion of the autumnal bowing, which Columella estimates at thirty days. Within this time, doubtless, the moveable "festival of seed-sowing" (-feriae sementivae-; comp. i. 210 and Ovid. Fast, i. 661) uniformly occurred. This month of rest must not be confounded with the holidays for holding courts in the season of the harvest (Plin. Ep. viii. 21, 2, et al.) and vintage.
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Reference address : https://www.ellopos.net/elpenor/rome/3-12-management-land-capital.asp?pg=10