Galatians 4:9-10 is a favorite crutch of those who claim Christians no longer need to observe God's holy days. However, Paul's meaning is quite different.
We can glimpse Gnosticism in Paul's epistles to the Galatians and Colossians, in which he combats Gnosticism's twisting of the truth of Jesus Christ.
The Colossian Christians were criticized by ascetics for the way they were keeping the Sabbath and holy days. Paul argues against a philosophy, not the law of God.
God has sanctified no day other than the Sabbath. Sunday worship is a pagan deviation, perpetuated by Gnosticism, a movement that despises God's laws.
Jesus Christ and God the Father are one in spirit and purpose, purposing to draw us toward that same kind of unity that currently exists between them.