This is the 214th most frequent Japanese word.
祭
Festival.
祭り refers to a festival or celebration in this context.
夏祭りに参加しました。
I participated in the summer festival.
祭 is used here to describe a ceremonial occasion or festivity.
彼らは感謝の祭を行いました。
They held a festival of gratitude.
祭 refers to cultural or traditional festivities in this sentence.