Some christian sects do not consider Sunday to be the Lords Day and they revert back to the Jewish tradition of celebrating the Sabbath on Saturday.  Catholics and the majority of Protestants celebrate the Lords Day on Sunday since this is the day on which Christ rose from the dead. This practice does not go against scripture as some would have you believe.  

“Therefore let no one pass judgment on you in questions of food and drink or with regard to a festival or a new moon or a Sabbath.”  (Col. 1:16)

“On the first day of the week, when we were gathered together to break bread, Paul talked with them, intending to depart on the morrow; and he prolonged his speech until midnight.”  (Acts 20:7) The first day of the week is Sunday!

“You observe days, and months, and seasons, and years!  I am afraid I have labored over you in vain.”  (Gal 4:10-11)