The Delphic festival of Megalartia is associated with the cultus of Demeter and involves offerings of large loaves or cakes. A festival by this name is also celebrated in Delos and Eleutherna, but dates can vary based on local parameters, with autumn celebrations in Delos and spring celebrations in Delphi. The core purpose of the ritual is to propitiate Demeter for the successful completion of the agricultural cycle and the growth of grain.
A spring festival that falls between Dioskoureia and Herakleia. The date selected is in this case quite arbitary, though both the greater and lesser mysteries run across the 21st of their months.
The timing of the rituals is not completely clear, but given the months at Delphi and in Thessaly, it may have taken place in the Spring or Summer. The cakes called δόλπαι which are offered to the goddess are attested as offerings on Kos, according to a lemma of Hsch. s.v. δολβαί· θύματα, οἱ δὲ (Κῷοι) μικρὰ πλακούντια. In other words, these were small or miniature versions of other cakes called πλακούντια. A πλακοῦς was a cake probably shaped like a mallow-seed (see LSJ s.v.); for further examples of these offerings, see here CGRN 201 (Miletos), line 37.
– A Collection of Greek Ritual Norms (CGRN) – Fragments from the sacrificial calendar of Eleutherna $^{57}$
In addition, fluctuation in festivals date according to local parameters is observed in the case of the Megalartia (Nilsson 1906, 235; Kravaritou 2006, 309-310), festival held in honor of Demeter that “brings loafs of bread”: beginning of autumn in Delos (Nilsson 1906: 316-317), and spring celebration in Delphi (Rougement 1977, 58-60).
[58]
assuming that this feast ("of the big loafs") may be assimilated to the Thessalian homonym, it must be assigned to the Demetriac cult, what is in accordance with the season of the year [15]
The Megalartia were an agricultural festival in honour of Demeter with this epithet, literally Demeter "of the Big Breads". The same festival probably occurs in the sacrificial calendar of the city of Eleutherna CGRN 210, line B3, where the goddess receives "small cakes" (no doubt with a wish to make them "grow"). [5]
17. Sacred - Demeter, throw down grain on the (threshing) floor; good for cutting beams for houses and ships
– Hesiod
Lines B3: We cautiously adopt Stavrianopoulou's excellent suggestion of reading offerings of cakes to the goddess Demeter called Megalartos ("of big breads"). Megalartia are attested as part of the many festivals of the Labyadai at Delphi, cf. here CGRN 8, lines D10-11 as well as in Thessaly, where a month Megalartios is attested at Thebai (cf. IG IX.2 109a-b and 133) and a priestess of the goddess is known at Pherai (IG IX.2 418; other evidence comes from Delos: Athen. 3.109f; and Boiotia: Paus. 9.4.4).
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In any case, the idea of offering small, seed-shaped cakes to Demeter "of the Big Breads" is a highly interesting one: it may suggest that the core notion of the ritual was the propitiation of Demeter for the successful completion of the agricultural cycle and the growth of grain; with the benevolence of the goddess, then, small cakes would ideally become big breads.
– A Collection of Greek Ritual Norms (CGRN) – Fragments from the sacrificial calendar of Eleutherna $^{57}$
On the present occasion, it wasn't Artos (Bread) that was wanted, but the loaves invented by Demeter, our Lady of the Grain and of Abundance. For with these titles the goddess is honoured in Syracuse, as the same Polemon remarks in his work on Morychus. And in Book I of his Reply to Timaeus he says that in the Boeotian town of Scolus B there are images enshrined of Megalartus and Megalomazus."
– Athenaeus Deipnosophistae
****https://penelope.uchicago.edu/Thayer/E/Roman/Texts/Athenaeus/3D*.html
Achaenae loaves.43 — This bread is mentioned by Semos in the eighth book of the Delias.44 He says that it is made in honour of Demeter and Korê. They are large loaves and a festival called Megalartia is celebrated by persons who contribute it reciting the words, 'a goat full of lard for our Lady of Sorrows.’*
– Athenaeus Deipnosophistae
****https://penelope.uchicago.edu/Thayer/E/Roman/Texts/Athenaeus/3D*.html