1Cor 16:1-3
"Now concerning the collection for the saints: as I directed the churches of Galatia, so you also are to do. On the first day of every week, each of you is to put something aside and store it up, as he may prosper."
The collection for the poor in Jerusalem (Rom 15:26; 2Cor 8-9) is the first reference to worship on the first day of the week as an established practice. The principle: systematic ('on the first day'), proportional ('as he may prosper'), voluntary ('put something aside').