A better way to think about it is according to two things that you find in Canon Law.
First, a day is a day is a day, midnight to midnight.
Can. 202 §1. In law, ***a day is understood as a period consisting of 24 continuous hours and begins at midnight ***unless other provision is expressly made; a week is a period of 7 days; a month is a period of 30 days, and a year is a period of 365 days unless a month and a year are said to be taken as they are in the calendar.
Second, the obligation to attend Mass on a day of obligation may be fulfilled either on that day or the evening of the previous day. It is not as if a day of obligation is some kind of “super day” that’s longer than 24 hours (laying aside the liturgical concept of “vigil” for a moment, and just looking at the legal concept of a “day.”)
Can. 1248 §1. A person who assists at a Mass celebrated anywhere in a Catholic rite either on the feast day itself or in the evening of the preceding day satisfies the obligation of participating in the Mass.
In other words, if there is an obligation to attend Mass on Sunday (a day that begins at midnight and ends at midnight, and is 24 hours long), it may be fulfilled on the 24 hours that is Sunday, or else on the evening of the day before (Saturday, a day that begins at midnight and ends at midnight, and is 24 hours long).
A day of abstinence or of fasting and abstinence is governed by this canon:
Can. 1251 Abstinence from eating meat or some other food according to the prescripts of the conference of bishops is to be observed on*** every Friday of the year*** unless a Friday occurs on a day listed as a solemnity. Abstinence and fasting, however, are to be observed on Ash Wednesday and Good Friday.
Notice that it says nothing about the evening of the previous day, as does the canon on Mass attendance. It simply specifies the days, that is, the 24 hour periods, on which someone is to fast or abstain from meat. And note as well that in most dioceses in the United States, there is no obligation to abstain from meat on Fridays, although some other penance is still encouraged.