@hightor,
We Germanic people – sorry, our Germanic ancestors – measured time in nights rather than days (e.g. ‘twelve nights’ instead of ‘twelve days’).
This ancient Germanic calendar, which counted by nights, originally began the day in the evening, thus extending the evening and night into the following day.
This old method of calculation gradually fell into disuse in the early Middle Ages, and by the 9th century, the evening was also referred to as belonging to the same day with the Old English word
aefented.
However, the old designation remained in use for holidays throughout the Middle Ages. Thus, the entire day before a holiday was often referred to as the “evening” of the holiday; hence Modern English Saturday.
Some remnants of this expression have survived to this day: modern German Weihnachts- or
Christabend, modern English
Christmas Eve, modern Scottish
Halloween, modern Danish
juleaften, modern Swedish
julafton, etc.
The fact that, in the Old Germanic calendar, evening and night counted as part of the following day also gave rise to some of the names of the days of the week:
i.g. “Friday”, old English
Frígeǣfen ='Thursday evening',
or “Sunday”,
Sunnanǣfen = “the eve of Sunday” = “Saturday evening, Saturday”;
Sunnanniht - 'the night from Saturday to Sunday.