The Catholic Church traditionally has rules which are strict, but which are interpreted with generosity.
Liturgical Sunday traditionally ended at midnight, when it came to Masses. People fasted for Mass beginning at midnight. There were some Masses said in the middle of the night for theater and newspaper workers (2-5 AM, usually), and that was totally okay for Sunday.
The Communion fasts got shortened to three hours and then one hour before receiving Communion, but Sunday still began and ended at midnight.
So when the Pope decided to allow Vigil Masses to fulfill the Mass obligations, and thus to extend Sunday all the way back to Saturday evening, he generously extended Sunday back.
However, as part of the generosity principle –
– you can go to Mass at night until midnight and have it count for that day (as for Sundays and Holy Days of Obligation)
– OR you can go to Mass from evening onward, and have it count as Mass for the next day (for Sundays and Holy Days of Obligation).
Every evening, from 4 to midnight, counts either way. Since we don’t have to fast from midnight on, we have a good chance of being able to receive Communion frequently, if we go to Confession and stay in a state of grace. This is Mother Church being nice to us.