Etymology probably refers to the slaying of bulls. https://translate.academic.ru/Βουκάτια/el/xx/.
The Boukatia is a Delphic festival in the month of Boukatios, dedicated to Zeus Patroos (ancestral Zeus) and Apollon. Not much is known about this festival, but it traditionally involved an offering of first fruits to Apollon, and a symposium, as well as a likely offering of oxen.
It was an important festival in Delphi, especially to ancient associations such as the Labyadai who swore their oaths before Zeus Patroos. There are possible connections between the Bouphonia in Athens (our favourite axe festival) and the Bouphonion in Delos. The festival may be associated with purification, with the swearing of oaths, and with renewal.
The exact date of the Boukatia festival is tied to the second month of the local calendar, with 10 Boukatios being the most likely date for the celebration, as an assembly of the Labydae is attested on this date$^{5}$. However, 9 Boukatios, cannot be ruled out entirely, and often appears in correlation with Dionysos who has a strong association with bulls.
The Boukatia festival is mentioned in the context of the Labyadai phratry in Delphi. The festival is associated with the month of Boukatios, which is named after the festival itself. It is likely that the Boukatia festival was a significant event in various city-states, although direct attestation is limited to Delphi. The festival involved offerings and sacrifices to Zeus Patroos and Apollon, likely including oxen, and featured a symposium for collective drinking. The term "akrothís" is mentioned in association with it, which could refer to first-offerings of agricultural produce. Some scholars suggest a possible connection with Bouphonia-type festivals, which involve the ritual slaying of an ox$^{62}$.
Boukatia was likley a significant event celebrated in various city-states. Greek months are named after festivals, and a month called Boukatios occurs in the Aetolian calendar used throughout the Aetolian league and in western central Greece. Boukatia, then, was likely the name of a major festival in these city-states, but the only festival directly attested is the one in Delphi.
While its specific rituals and customs are not extensively attested, the festival involved offerings and sacrifices to Zeus Patroos and Apollon. The Labyadai celebrated this festival with reverence, likely sacrificing oxen and presenting first-offerings of agricultural produce. The festival also included a symposium, providing an opportunity for communal drinking and enjoyment.
Zeus Patroos and Apollon, who receives the first fruits; in the same month the Labyadae have a nomimos thoina (D,3/4) and "drink all together"
Emilio Suárez de la Torre, Neoptolemos at Delphi $^{15}$
Line D4: Following Dittenberger, Rougemont (p. 58) notes that the Boukatia will almost certainly have fallen on 10 Boukatios, the second month of the local calendar, since an assembly of the group is attested on this specific date (lines A19-21 in the preamble of the decree; cp. A41-42; 9 Boukatios, the day preceding the meeting, is also possible). As is clear from lines D43-51, the group offered sacrifices to Zeus Patroos and Apollo on this day, perhaps including oxen given the name of the festival, but notably featuring, according to lines D43-51, an offering of first-offerings (τὰν ἀκροθίνα) and a symposion for collective drinking.
For the cult of gods called by the epithet "ancestral", i.e. patrooi, see here e.g. CGRN 18 (Thasos); for first-offerings, probably of agricultural produce, cp. here CGRN 60 (Thera), lines 13-15, and CGRN 61 (Athens), line 1. As Rougemont notes (p. 59 with n. 186), the date of the Pythia at Delphi was also in the month Boukatios, but the occasion celebrated by the Labyadai in this case was almost certainly a distinct festival.
– A Collection of Greek Ritual Norms (CGRN) – Two excerpts from the dossier of regulations of the Labyadai at Delphi $^{5}$