steve-b:
That’s why we can have a Saturday vigil mass that begins at sundown Saturday and have it count for Sunday mass.
I know this is often said, but it’s not really the true reason we can have Saturday evening Mass. If it was, then we would not be allowed to have mass on Sunday nights.
The reason we can have Sunday mass on Saturday after vespers is because all Solemnities historically have a vigil (even if it doesn’t have its own Liturgy).
After Vatican II, all Sundays were raised to the same level as a solemnity. Therefore, because all Sundays are equal in Rank to a solemnity, it was argued that they should be allowed to have a “vigil” like a solemnity.
Also, before this was allowed, it wasn’t uncommon for a Parish with a population with early Sunday workers to have a 2am 4am or 5am Mass on Sunday. I’m sure some places even had a midnight mass.
Some city Parishes had a “newspaper workers’ mass at 2am for people who had to be working at the newspapers by 4 or 5am
So the Saturday vigil mass was created to (1) allow those people who work Sunday mornings (and the priests) to attend mass at a more reasonable hour (and with families).
Note Vatican II also changed the Liturgy of the Hours by no longer requiring the priests to wake up around 2am to pray the Matins (now called the Office of Readings). So before VII, some priests had no issue having a 2am Mass because they were already up.
Anyway, I’m sure there are far more details, but that’s the real reason we can have Saturday evening mass