Under the old covenant, the last day of the week commemorated a finished creation. (Physicality)
Under the new covenant, the first day of the week commemorates a finished redemption. (Spirituality)
Under the old covenant, under the authority of Moses, God’s law was obeyed physically.
Under the new covenant, under the authority of Christ, God’s law is obeyed spiritually.
Saturday represents the old covenant.
Sunday represents the new covenant.
Jesus rose from the grave on a Sunday morning- His resurrection means we can join Him in resurrection. The early church met together daily, but it is often mentioned that upon the first day of the week they did things such as- assembled together, laid by and store (give to God), listened to preaching, partook of the Lord’s Supper in the assembly of the saints- sounds like church to me. Therefore, even though I go to church on other days of the week, the first day of the week is an absolute must.