Who decides what day is the first day of the week? If the first day of the week somewhere else is not Sunday, what day do they worship upon?
God has decided what the first day of the week is by deciding what the seventh day of the week is. That is, the first day of the week is defined in terms of the seventh day. We are familiar with the creation account in Genesis 1 that says that God made everything in six days and He rested upon the seventh day. That seventh day then became the Sabbath day of Exodus 20:8-11 “Remember the Sabbath day, to keep it holy. Six days shalt thou labour, and do all thy work: But the seventh day is the Sabbath of the LORD thy God: in it thou shalt not do any work, thou, nor thy son, nor thy daughter, thy manservant, nor thy maidservant, nor thy cattle, nor thy stranger that is within thy gates: For in six days the LORD made heaven and earth, the sea, and all that in them is, and rested the seventh day: wherefore the LORD blessed the Sabbath day, and hallowed it.” The Sabbath day is Saturday. So the first day of the week must be Sunday as it is the day after the Sabbath. God is the one who decides what the first day of the week is.
The second question is a little more difficult. First let me say that I cannot think of a non-hypothetical situation where we wouldn’t know what the first day of the week is. Today, the daily calendar is pretty much set worldwide. International travel has pretty much set the standard in every single country in the world. So, the second part of this question can only be answered hypothetically. Let’s suppose that we were shipwrecked on a desert island in the middle of the Pacific Ocean. Having been washed up on shore and been unconscious for several days, we have completely lost track of the day of the week. As soon as we wake up, we wander inland and find a church building with a lifetime supply of unleavened bread and fruit of the vine, so we determine that we are going to worship God. How are we going to determine which day is Sunday? Genesis 1:14 says, “And God said, Let there be lights in the firmament of the heaven to divide the day from the night; and let them be for signs, and for seasons, and for days, and years.” So, using the information from the sun, moon, and stars, we should be able to determine which day is Sunday. Astronomers are able to do this today and if we ever get into such a situation, it might take some time, but we could eventually figure it out. I guess the bottom line is that we are always going to be able to determine which day is Sunday one way or the other. Currently, there are not any places that I know of where the first day of the week is not Sunday.
We should note that we are privileged in our society to have the first day of the week off from labor. In the time of the early Christians it was not so. The Jews took off Saturday to observe the Sabbath, but the first day of the week was a working day. The gentile world knew nothing of days off and if you were a slave, you would be lucky to get a decent night’s sleep. Early Christians sometimes met in the evenings after the work day was finished. The best example of this in the New Testament is in Acts 20:7-12. Paul met with the disciples in the evening. Poor Eutychus probably had just gotten off of a hard days work and sitting in that upper room with all of those oily candles burning made him sleepy. So he sat in the cool of the window probably to try and stay awake. However, it just was not enough. Fortunately, Paul was there to bring him back to life. We are indeed fortunate to have buildings, padded pews, incandescent and fluorescent lighting, heaters, and air conditioners. We are also fortunate to have the first day of the week off from our labors. Such has not always been the case for Christians.