The Sabbath is Saturday. That has never been officially changed except by Edict of Constatine, at the Nicean Council. The main reasons given for the change were because the Jews do that, so we won't.
The apostles met immediately after Sabbath, called havdalah, Saturday night.
And they would not collect money on the Sabbath, because this was against Torah, so they met at havdalah.
However, thankfully we live in grace. This not a license to sin, but if some are wrong about Sabbath, they are still "saved".
When Paul was writing to those regarding new moons and holy days, he was talking about their customs, not the customs of the Jews, because they were Gentiles.
Bruce
Bruce