There is a good description of the Feast of the New Yam in Chapter Five. This feast is meant to thank and praise the goddess of the earth, Ani, who is responsible for "all fertility" (36). The feast begins before the actual harvest because it is important to thank the goddess before reaping the benefits she has offered. Ani is offered some of the new yams, and everyone must get rid of all the old yams. Cooking and eating pots and utensils are cleaned thoroughly, I think because the old is not supposed to mix with the new. This is the beginning of a new year. The primary dishes are yam foo-foo and a vegetable soup, and food is cooked in great quantities. Hospitality is strongly encouraged. On the second day of the feast, the village of Okonkwo and a nearby village hold a wrestling match.
This harvest tradition has many of the same elements as harvest traditions all over the world, fixing a new year to the harvest, giving thanks for what is bestowed upon the community, customs to create a clean slate, whether literally or figuratively, an emphasis on bounty, and a tradition of hospitality. In fact, it reminded me strongly of my own Jewish New Year of Rosh Hashonah and Yom Kippur.
As you know the Yam Festival is a popular holiday in both Ghana and Nigeria. After offering yams (the most common food in many African countries) to the the appropriate deities and ancestors, the processed yams are distributed to local villagers. The Festival is one way residents of the villages give thanks to spirits (ancestral and earth goddess) they honor and worship.
As a successful high school student, you could make rich connections to traditions of "giving thanks" familiar to you and practiced by you and/or your family traditions.
Obviously the Festival has particular significance to the theme and Okonkwo's conflicts present in Chinua Achebe's Things Fall Apart.