On Sundays and solemnities one is not obliged to fast. I have heard that these days start a bit earlier, liturgically speaking, namely on the evening of the preceding day. So from what point is one no longer obliged to fast -- First Vespers (for those who pray the Liturgy), sunset, or midnight?

Midnight to midnight is what's in canon law. There is no support whatsoever for letting people out the evening before.

And what, exactly, is a fast? I would like to hear from those who know.

The law of fasting allows only one full meal a day, but does not prohibit taking some food in the morning and evening, observing—as far as quantity and quality are concerned—approved local custom.

Moralists generally hold that any liquid that is meal like taken besides at the main meal or as part of the two "snacks" [collations] breaks the fast. So no milkshakes!

The Eucharistic "fast" excludes everything but water and medicine for an hour (formerly it was 3 hrs from food and alcohol, and 1 hr from every other beverage except water, and prior to 1955 it was from midnight to communion, and even water broke the fast)

A liturgical day is midnight to midnight according to the De calendario section of the Missal. Arguably Easter Vigil is an exception. Back in the day when we fasted for an actual 40 days, the Lenten fast ended with the Vigil, even if that was over before midnight. But then again back in the day the Vigil started in violet, but ended in white to show the change...it was a "sui generis thing" as far as that goes

But the new liturgical week begins on the eve of Sunday. Evening prayer on Saturday already belongs to Sunday, liturgically speaking; the closing prayer is that of Sunday. Yet the liturgical day is still Saturday?

(And I should have said "abstain" rather than "fast", sorry.)

There is no such thing as a "liturgical day" in the sense you are using it. A day is midnight to midnight. Under some circumstances, the Church allows a Mass celebrated the evening before to satisfy the obligation for the next day, and allows the propers for that day to be used in advance. But a day is a day.

It's not just that a Mass may be celebrated earlier, but also that Vespers on the eve of a Sunday or solemnity have the closing prayer of the next day. Also, as I mentioned, the breviary begins the new liturgical week with Sunday eve.

Still, you might be right that I'm misunderstanding the concept of liturgical day. In that case I've been doing it wrong this Lent, but I'll change it next year.

I am not sold on the concept of "liturgical day" anyhow, but since the obligation to fast is in canon law, and since canon law clearly specifies that a day is midnight to midnight unless something else is explicitly stated, the fast runs on canonical days.

Moreover: Celebrating the anticipated Mass on Saturday is universally agreed not to count against Sunday's Mass total, allowing a priest to celebrate three Masses on Sunday even if he's celebrated an anticipated mass (or two) the night before. Therefore Saturday is not part of the Sunday "liturgical day."

That makes sense. Thank you!

On second thoughts, the Code of Canon Law in its section on Days of Penance (can. 1249-1253) does not even explicitly state that Sundays during Lent are not penitential days; this might be implied in can. 1247 (where the faithful are required to abstain from everything that hinders "the joy proper to the Lord's day") but is not clearly stated. So whether or not one relaxes one's Lenten penance on Sunday seems to be a personal choice; it is not regulated by canon law.

In that light, considering the fact that the Sacred Congregation of Divine Worship has said that "the observance of Sunday and solemnities begins with the evening of the preceding day" (http://www.catholicculture.org/culture/library/view.cfm?id=5932#Days), wouldn't it be fitting to relax one's penance on the preceding evening?

If forced to take a side in such an argument, I would argue that Sunday doesn't relax the observance.

What penance can you possibly be doing that is so severe that you have to find an excuse to do it on Saturday evening?

The ancient Lent fast was exactly 40 days (after Ash Wednes day and ff were added), we fasted Monday-Saturday, and abstained from meat Sunday through Saturday. When violet was introduced into the liturgy, it was first used on Sundays of Lent, leaving black on weekdays (a practice still used in Milan!). Mutandi mutatis. I would argue that Sundays, while still penitential (we abstained from meat traditionally) are not as penitential as the weekdays. Formerly Lent was equivocal referring to the 40 days fast (that did not include Sundays) and the season of the 40 days (including Sundays). Now it is redefined and isn't 40 days and what not. But still, as a rule of thumb I think a stricter observance on weekdays and a lighter observance on Sunday makes sense and "fits" with tradition and the Paschal character of Sunday

Though if your penance is basically all or nothing thing, I err in keeping it on Sunday. I had a friend who gave up buying cigarettes during Lent...but he bought them on Sunday! And then he bummed on Friday/Saturday if he ran out. His original idea was not buying cigarettes = only smoking in social situations. But the Sunday thing messed with that and made it moot!

Sometimes it's just useful, such as when the family goes to a restaurant on Saturday evening. Sometimes it's simply the weakness of the flesh. I still need to cultivate the right disposition of fasting. Hopefully next year will be better.

Anyway, whether or not Saturday evening belongs to Sunday (and whether penance is fittingly relaxed on Lenten Sundays) doesn't matter to me; I just want to do the right thing. Last year I was told by a seminarian that Sunday started with First Vespers and I had no reason to doubt him. Now that the matter has become doubtful, I'll do some further research.