That was something that was decided on by the first Christians, and it is Biblical. They called it the “Lords Day”. Here are two verses showing that they had Mass (broke bread) on Sunday (first day of the week):
[color=darkred:1ilwbksa]And on the [u:1ilwbksa]first day of the week[/u:1ilwbksa], when we were assembled to break bread, Paul discoursed with them, being to depart on the morrow. [/color:1ilwbksa]Act 20:7

[color=darkred:1ilwbksa]Now concerning the collections that are made for the saints: as I have given order to the churches of Galatia, so do ye also.
On the [u:1ilwbksa]first day of the week[/u:1ilwbksa], let every one of you put apart with himself, laying up what it shall well please him: that when I come, the collections be not then to be made[/color:1ilwbksa].1Co 16:1-2

And here is a reference from the very early Church:

[color=green:1ilwbksa]If then those who had walked in ancient practices attained unto newness of hope, [u:1ilwbksa]no longer observing sabbaths [/u:1ilwbksa]but fashioning their lives after [u:1ilwbksa]the Lord’s day[/u:1ilwbksa],[/color:1ilwbksa]
Ignatius, To the Magnesians, 9:1 (A.D. 110)
http://www.earlychristianwritings.com/t … tfoot.html

[color=green:1ilwbksa]But every [u:1ilwbksa]Lord’s day[/u:1ilwbksa] gather yourselves together, and break bread, and give thanksgiving after having confessed your transgressions, that your sacrifice may be pure.[/color:1ilwbksa]Didache, 14 (A.D. 90)
http://www.earlychristianwritings.com/t … berts.html