Please note that in the Catholic Church Sunday begins after First Vespers on Saturday.
This is normally assumed to be around four o’clock which is when in cathedral churches the choir sings First Vespers.
This is why it is possible for Catholics to fulfill their obligation to attend Sunday Mass on a Saturday evening.
This also applies to major feasts that are not on a Sunday
e.g. The Assumption Sunday - Mass 11.30 am - Weekday and Seasonal times may change