Shabbat 35 - Twilight
Twilight on Friday afternoon might be still Friday, a weekday, or it might be Shabbat. At this time, one should not separate the tithes from his produce. Ordinarily, one is allowed snacks before he separates tithes, but not a complete sit-down meal. However, on Shabbat even snacks have a feeling of restful unhurried meals, and thus untithed produce cannot be eaten at all. Even though tithing is not really work, it is prohibited by the Sages because it is too similar to a repair: the food was not allowed to be eaten, and now - it is! Neither should one dip his dishes in a mikveh to purify them at this time - again, because it is similar to repairing them.
Those types of work that are only prohibited by the Sages and are too far removed from actual Shabbat violation can be done at twilight - such as, for example, wrapping the food to keep it warm. Why would wrapping not be allowed on Shabbat itself? Because one might find that the dish has cooled off somewhat, and warm it before wrapping, and that would be the prohibited labor of cooking. If so, we should not permit it even during twilight!? - No, during twilight all dishes are piping hot, so he will not warm them up.
How long is twilight? There are many possible definitions, ranging from the time it takes a person to walk 8,000 steps, all the way down to a moment so short that one cannot determine it.
If one has performed work at twilight both on Friday and on Saturday afternoons, he has to bring one sacrifice, regardless of how you look at it: if twilight is still day, he is liable for working during twilight on Shabbat, and if it is night, then he is liable for work done at twilight on Friday.
Art: Winslow Homer - Twilight at Leeds, New York