Rabbi Dr. S.M. Lehrman is the author of the book Our Festivals. In his article ‘Festivals’ in the 1938 Jewish Encyclopaedia,1 he writes about the "pilgrimage festivals [of] Passover, Pentecost and Tabernacles (Exodus 23:14)" – that "they were originally pastoral in character, marking the Spring (barley), Summer (wheat) and Autumn (fruit) harvests respectively.
"Adult males…had to appear in the Temple, bearing gifts showing their gratitude to God Who blessed the Seasons with fruitfulness (Deuteronomy 16:16). This pilgrimage…is referred to in the Bible also as Moade Adonai [alias the Lord’s Feasts], because of their celebration on fixed specified days….