@hristov it’s not really true. Christians began to celebrate Sunday instead of Saturday  since the time of the apostles. Paul says we met together in the upper room and broke the bread on the first day of the week(the Lord’s day). It was not a tradition from Constantine for the gentiles. That’s a typical secular explanation which isn’t remotely true. Sunday was regarded as the “Lords day” by the apostles because it was the day Jesus was resurrected on. 

Acts 20:7 

@Dušan is correct. The days of the week most likely came from st.cyril and Methodius who came and ministered to great Moravia. Their disciples later went back to Bulgaria after the fall of great Moravia and Bulgaria later christianized the Kievan Rus (Eastern slavs). So technically it did come from one source namely from Greece/Bulgaria.