The first Christians were Jewish, so they naturally observed the Sabbath on Saturday--just like they always had. They worshiped the God of Abraham, Isaac, and Jacob--just like they always had, but they did so with the realization that the God of Abraham, Isaac, and Jacob and visited them, that is, that Jesus the Messiah, the eternal Son of God had become a man, had been crucified, buried and rose again on the third day, that is, on Sunday.
It also appears that they began to observe the first day of the week as Jewish believers in order to honor the day of the resurrection.
On the first day of every week, each of you is to put something aside
and store it up, as he may prosper, so that there will be no
collecting when I come. 1 Corinthians 16:2 ESV
Gentiles were never required to become Jewish in order to follow Jesus. They were not required to be circumcised or to observe the Jewish Sabbath or refrain from eating pork--or any of the covenant laws of Israel.
Still, Paul and Peter continued to attend Sabbath days in the synagogue and to proclaim Jesus.
The verse immediately after the one you cited states this:
These are a shadow of the things to come, but the substance belongs to
Christ. Colossians 2:17 ESV
So, Paul is stating that the New Moons, Sabbaths and all that (from the Law) were a shadow of the things that were to come. The reality is found in Jesus. Therefore, if Gentile believers did not do all of these things, they should not be concerned with the Judaizers or any who would criticize them for that.
As Gentile believers in Jesus the Messiah, it seems appropriate to honor the day of resurrection, following the pattern set forth by the early church.
Still, if you happen to celebrate and worship Jesus on the wrong day, I'm sure that's alright. In fact, we should probably worship Him every day of every week, and certainly not just one day a week.