Some people go to church on Sunday....and Wednesday.....and Saturday......and any other days that the church meets.....one day at least needed to be agreed upon, and Sunday just so happened to be it.....also Sunday School has a much better ring to it than Thursday School......and Sunday, kind of a weird word, Sunday, as if it's always sunny on Sunday or something......
The Creator God used the seventh day as a memorial to His six days of creative activity and asked us humans to honor it weekly in memory and respect of His work. Even Jesus honored his Fathers wishes by keeping the seventh day holy plus all the other Commandments as the fifth one required.
At creation God created everything in six days and rested on the seventh day and hollowed it as a day of rest. On 7 March 321 emperor Constantine decreed that the new day of rest would be sunday as it was the day Christ rose from the tomb, calling it the venerable day of the sun. Constantine was still a pagan sun worshiper, and only converted on his death bed.
Simply because Sunday is the day we celebrate the Lord's resurrection from the dead. The third commandment states: "Remember the Sabbath day, to keep it holy" (Ex 20:8; Dt 5:12). For the Jewish people, the Sabbath falls on the seventh day of the week, Saturday, the day God rested from creation. The Christian church has always, since the days of the apostles, celebrated the Eucharist on Sunday as a way of commemorating Jesus' resurrection (see Catechism of the Catholic Church 2177-2179).