Does the new day start at midnight and end at 11:59 pm? Is it from the time you get up one morning until the time you get up the next? Or do you follow ancient traditions and count from sundown to sundown?
Most Catholics have probably contemplated this question on at least one Friday night, when the clock says 10 pm, the sky is dark, and they are suddenly craving meat. For Catholics, there is no hard and fast rule on when one day ends and the next begins for religious purposes. The best advice I've heard is that we should decide for ourselves which measure we are going to employ, and then stick to it: Saying that Friday began and ended at dark last week because the pizza place was conveniently open several hours after dark, and then changing it this week so that Friday begins and ends at midnight so you can have that late-night pie on Thursday, is just plain cheating, and only a crazy person thinks that they can get away with cheating God.
In this part of Alaska, though, we can generally discount "sundown to sundown" as an option: In the middle of winter, the sun rises at or after 10 am and sets by 3 pm. In the summers, we eventually make our way up to nearly 24 hours of daylight, so there isn't even a "sundown" to speak of. Today the sun rose at 3:31 am, and it will set at 12:13 pm tomorrow. If we tried to keep the "sundown to sundown" rule here, over the course of a year we would be observing "Friday" at very different times depending on the season!
Our family uses the "getting up one morning to getting up the next" measure. While we are almost never up late enough that it would be different than basing the determination on what the clock says, it certainly makes things easier when we visit friends and family out of state: Alaska is in a time zone all to itself, so no matter where we travel, we are always gaining or losing at least one hour. Being stuck in an airport in the middle of the night is difficult enough, without having to figure out whether or not it is technically a day of fast or abstinence in that particular time zone when the only restaurant open is McDonalds...
How do you measure Fridays?