Islam, I believe has the Sabbath on Friday. When in Jerusalem at the Wailing
Wall, I believe
Fridays would be when the Muslims would come pouring out of the Mosque at the
Dome of the Rock and often start trouble.
I'm not sure about Islam, but the Jewish Sabbath is on Saturday because Jews
believe God rested and blessed the seventh day. Christians believe Jesus
rose on the first day of the week, that's why our day of worship is Sunday.