Saturday, according to the Convert’s Catechism of Catholic Doctrine:

Convert’s Catechism of Catholic Doctrine
Q: Which is the Sabbath Day?
A: Saturday is the Sabbath Day.

Convert’s Catechism of Catholic Doctrine, Peter Giereman, 1948, p.50
https://www.scribd.com/document/395994377/The-converts-catechism-of-Catholic-doctrine-pdf