You have to put a dividing line for anticipated Sunday Masses somewhere.
- Would you have all Masses on Saturday count for Sunday?
- Where would you put a Saturday daily Mass?
- Why stop at Saturday?
Why not make every Mass during the week the same and count for the Sunday liturgy?
The point of anticipated Masses is to make it easier for those who have odd schedules to fulfill their Sunday obligation by stretching the liturgical day to accommodate them. You can only do this so far before it gets ridiculous. Admittedly, 4:00 pm is somewhat arbitrary — it could just as well have been 5:00 pm or 3:00 pm — but it is a reasonable choice for the boundary.
If your question is simply why is it 4:00 pm and not 5:00 pm, 3:00 pm, or 3:37 pm, I don't know, but 4:00 pm seems to be a good compromise. The choice of time frame, roughly speaking, is related to the Jewish custom of accounting the day as beginning at sundown (the night before) instead of midnight. But of course sundown varies from place to place and month to month and perhaps they thought it would be better to set a fixed time.
It's sort of (though not quite) like the cop who opts not to pursue you if you're 5 miles over the speed limit. That's a reasonable margin.
- But why 5 miles over the speed limit?
- Why not 10?
- Why not 15?
- Why not 20?
Because at some point you are just defeating the purpose of the law by stretching it beyond what is reasonable.
While this example differs from anticipated Masses, in that anticipated Masses are provided for in the legal code, a similar principle applies. The point of Sunday liturgy is to sanctify the Lord's Day (Sunday). Thus it has to have a close relationship to the Sunday calendar day. 4:00 pm is close enough to Sunday to sanctify it but once you move further back in the day or week, you're basically sanctifying Saturday, which is the wrong day. It becomes harder and harder to argue that you're sanctifying Sunday the further back you go in the week.
Anticipated Masses are a relatively recent introduction . . . within the last half-century. Anticipated Masses are a gift of generosity; a magnanimity; a grace.
The Church is being pastoral in allowing them. Let's accept the gift gratefully.