That would be Saturday, The Sabbath as all the Apostles did.
After the resurrection of Jesus, when the old covenant began to pass away from the scene (Heb. 8:13), where did the apostles gather with the people and set aside the last day of the week? Please be specific.
On the other hand, we do have examples of the church gathering on the first day of the week to break bread and hear the apostles teach. (In spite of the fact that you wish to twist the Scriptures to make "first day" read "last day".)
So you are the one with practices out of sorts with the new covenant. I'm sure the Judaizers had objections similar to yours.
If you continue reading in Hebrews through chapter 9 you will see what had become obsolete....certainly not the recognition and observance of God's Feast Days and Sabbaths. As mentioned in previous posts, the Levitical priesthood with all of the regulations and ceremonies had been replaced. See Hebrews 7:11-28. God's Holy Days and Commandments were not part of the priesthood. I truly am sorry you cannot see this....especially when those statuates and ordinances were given to us for all times.