We do know, because the Apostles, all Jews, would attend the synagogue on the Sabbath, and then they would celebrate The Lord’s Day on Sunday in little house churches since Christ has risen from the dead on a Sunday.
“The Lord’s Day “ is identified as what? Mark 2:28 gives a clue.
Hebrew days started at sundown. Sunday started at sundown on Saturday, the Sabbath. Believers would meet at sundown to celebrate the Havdalla ceremony or saying good bye to the Sabbath. It was then they would have a meal, sing Psalms, have a teaching and go home to get up the next day, which was Sunday to go to work. Jesus followed this every week of his life, the disciples followed this routine every week of their life. EVERYONE WENT TO WORK ON SUNDAY, the first day of the week. The Nicean Council 325AD declared Sunday the Sabbath. GOD NEVER CHANGED THE SABBATH.