Sunday, the “first day of the week”, where they “all gather on the day of the sun”:

Catechism of the Catholic Church – The third commandment
The day of the Resurrection: the new creation
2174 Jesus rose from the dead “on the first day of the week.”¹⁰⁴ Because it is the “first day,” the day of Christ’s Resurrection recalls the first creation. Because it is the “eighth day” following the sabbath,¹⁰⁵ it symbolizes the new creation ushered in by Christ’s Resurrection. For Christians it has become the first of all days, the first of all feasts, the Lord’s Day (he kuriake hemera, dies dominica) Sunday:
We all gather on the day of the sun, for it is the first day [after the Jewish sabbath, but also the first day] when God, separating matter from darkness, made the world; and on this same day Jesus Christ our Savior rose from the dead.¹⁰⁶

https://www.vatican.va/archive/ccc_css/archive/catechism/p3s2c1a3.htm#2174