Some imagine this verse describes Sunday services where the congregation drops their offerings in the collection box when it is passed around. But Paul is not telling the members to have worship services on the first day of the week and at the same time make offerings. The collection was for the brethren in Jerusalem that were experiencing famine (1 Cor 16:3; Acts 11:27-30).
Paul’s instruction to the Corinthian church to “lay something aside” was to facilitate collection when he came. The first day of the week is the day after the Sabbath, the day when they could start working and collecting the food. It’s also quite possible that he wanted the believers to set aside the donation at the beginning of the week so that it would not be consumed during the rest of the week.