The best evidence for these sacrifices is financial. Greek cities inscribed their accounts on stone and set them up in public, and among the surviving records are the costs of festival sacrifices: what was paid for the animals, which magistrates handled the money, and how much came back from selling the hides of the victims. The hides were a substantial and predictable revenue, handled by officials and recorded year on year, which means the number of animals killed at a given festival can sometimes be recovered from the receipts rather than from any narrative.
This has two consequences for how the rite should be read. The first is that these events were planned as public expenditure, budgeted and audited, and belong as much to the history of civic finance as to the history of religion. The second is corrective: because the accounts survive better than the ceremonies, modern reconstructions of Greek festival life lean on financial documents, and the picture they give is skewed towards what cost money and away from what did not.
The month name Hekatombaion, which opened the Athenian year, preserves the older sense of the word as the name of a particular great offering rather than a class of them. By the Classical period the sacrifice that gave the month its name was no longer its most prominent feature, and the name had become a fossil, which is a common fate for the vocabulary of sacrifice: the words outlive the rites and end up naming months, prayers and hours.