From the article:

Most modern people who know the myth of Persephone think that the ancient Greeks believed that she was in the underworld during the winter and with Demeter for the rest of the year. Even many professional classicists think this. I, however, like some other scholars, am convinced that this is incorrect. The surviving ancient sources for the myth are unclear about which part of the year Persephone spends in the underworld and it makes far more sense given everything we know about the ancient Greek agricultural and religious calendars to conclude that the Greeks believed that she was in the underworld during the summer, not the winter.

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Hesiodos indicates that the Greek agricultural calendar for grain during the period in which the Homeric Hymn to Demeter was composed was the exact opposite of the agricultural calendar throughout much of the western world in the twenty-first century; for the Archaic Greeks, winter was when the grain was growing and summer was the fallow period.

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If the ancient Greeks believed that Persephone was in the underworld during the winter and that she was reunited with Demeter in the spring, then we would expect them to celebrate the major festivals of Demeter and Persephone in the spring. Instead, both of the major Attic festivals honoring the two goddesses took place in the autumn. This suggests that the people of Attikē regarded the autumn—not the spring—as the time when Persephone returned from the underworld and was reunited with her mother.