The better question is why the Jews chose Saturday. Exodus, Chapter 16, if I recall correctly, notes that God provided a double portion of manna on the sixth day so that the people might rest on the seventh. Jews marked this day, and the Sabbath has been on Saturday ever since.
Christians: Jesus returned to life on Sunday, and this is the day Christians renew that new life within themselves.
Both days have great meaning for the people of each faith, and each should be honored equally.
This article may help understand what I'm saying:
Saturday Sabbath? Or Lunar Sabbath?
Here are a couple excerpts from it:
1. Scripture itself supports this calendar configuration. Every time the seventh-day Sabbath is given a numbered date in the Bible, it always falls on the 8th, the 15th, the 22nd or the 29th of the month.
This should be enough to help understand that the Sabbath was Lunar in ancient days like in Exodus and that the Sabbath would change each month, year and so on.
2.
Jews and the Lunar Sabbath
While it is true that the Jews today worship on a Saturday sabbath, this does not prove that it is the true Sabbath. They did not always worship on the Saturday sabbath. Jewish scholars are very clear that the original method of calendation was different from the modern calendar and that under intense persecution during the fourth century A.D., the Jews gave up their original lunar-solar calendar with its lunar Sabbath. Declaring the new month by observation of the new moon, and the new year by the arrival of spring, can only be done by the Sanhedrin. In the time of Hillell II [4th century C.E.], the last President of the Sanhedrin, the Romans prohibited this practice. Hillel II was therefore forced to institute his fixed calendar, thus in effect giving the Sanhedrin's advance approval to the calendars of all future years. ("The Jewish Calendar and Holidays (incl. Sabbath)": The Jewish Calendar; Changing the Calendar,
www.torah.org, emphasis supplied.)
Once the ancient lunar-solar calendar had been set aside, knowledge of the true lunar Sabbath was lost with the acceptance of the pagan Julian calendar.
Lunar Sabbath Lost