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