Right - it’s a tad bit confusing. The Good Friday fast/abstinence is midnight to midnight. But, when liturgically talking about the Triduum, it’s more like evening to evening. So. The fast/abstinence is midnight to midnight - BUT - liturgically, the Triduum is considered to be Good Friday, Holy Saturday, and Easter, not Holy Thursday, Good Friday, Holy Saturday. Why? Because it mirrors the Jewish feast of Passover (in fact, it IS Passover). And Jewish days are sundown to sundown.
It gets confusing, because Jesus’s Passover seder with his apostles (the Last Supper) took place on the same Jewish day as His Passion. But it was a different day on the Roman calendar (which went midnight to midnight). And so the Triduum, liturgically, follows the Jewish calendar, while the fast and abstinence of Good Friday follows the Roman calendar.
It’s also why Easter moves all over the place - it’s originally based on the date of Passover, but since the Church found it ridiculous to ask Jews when Passover was so that we could celebrate Easter (and because the Jews themselves couldn’t agree - besides, they would probably were offended themselves), we decided to calculate the date of Easter for ourselves (the differences between the Western Easter and the Eastern Easter have more to do with the difference between the Gregorian and Julian calendars on the exact date of the first day of spring). This is why Easter and Passover usually fall around the same time, but not always.
