https://www.crossway.org/articles/did-jesus-lie-to-his-brothers-john-7/

James M. Hamilton writes about Jesus’ encounter with His brothers in Jn.7 about going the the Feast of Tabernacles. I want to make another point about why His brothers would have understood that He is not saying He is not going at all, but, rather He will not go publically.

In order for Jesus to fulfill all righteousness, He had to observe all the Sabbaths and the feasts of Lev.23 (and elsewhere). Jesus’ brothers would have seen that every Sabbath and Covenental Feast Jesus observed since these were Old Covenant requirements. That Jesus was careful to fulfill the Law is seen in His baptism when John the Baptist wanted to reverse roles: Jesus replied, “Let it be so now; it is proper for us to do this to fulfill all righteousness.” Then John consented (Mt. 3.15 NIV).