In 1 Corinthians 16:2, on which day were believers to set money aside?

Answer: The first day of the week

For the collection, Paul said that on the first day of every week each should set aside a portion of his income.

“Now about the collection for the saints, you are to do as I directed the churches of Galatia: On the first day of every week, each of you should set aside a portion of his income, saving it up, so that when I come no collections will be needed.”— 1 Corinthians 16:1-2 (BSB)