Jews and pagans: what are the reasons for starting religious festival in sunset?

I think you're looking for extra meaning where there is not any. If you look into calendars used by various peoples around the world, about half of them start at sunset, and about half of them start at sunrise...so yes, it is probably just random. (that's excluding a few cultures who don't count night and day as part of the same thing, IIRC) Most Pagans whose liturgical days start at sunset are basing it on the calendar of the ancient Celts, who did not likely get the idea from any Semitic peoples.

BUT the Celts, Jews and Phoenicians (another Semitic people) also all have creation mythos that begin with a world in darkness. Then from the darkness comes light. It's the order of things...First dark, then light. Compare this to the Hellenic (and then Roman) idea that things start out in chaos (which literally means gas, or haziness, or grayness) and then get ordered.

Because the day ends at sunset, therefore, logically, the new 'day' would begin at sunset. Also... the night is like the womb from which all things procede... Much like your analysis in reason A.

There were no clocks telling us when "midnight" was back when... so people went by what they could see... the sunset was a definitive marker in time, not like the nebulous unseen hour of the middle of the night for which there are no natural signals.

-Scarlet

Sunset is when the next day in the Jewish Calendar Starts. So we basically start the festival at the beginning if the Jewish Calandar day and continue it until the next calendar day begins after sunset the next day.

I'm a Canaanite polytheist. In our calendar, our days begin at sunset. So our festivals begin at sunset. It's similar to the Jewish calendar.

I believe because ancient people didn't have artificial light, sunlight time was dedicated to field work, in barn whatever.And only after sunset they had little spare time.

Because from sundown to sundown encapsulated an entire 24 hour day.

And God said, “Let there be light,” and there was light. 4 God saw that the light was good, and he separated the light from the darkness. 5 God called the light “day,” and the darkness he called “night.” And there was evening, and there was morning—the first day.

See where it says and there was evening and there was morning- they start in the evening because they believe God made it that way.

Pagans celebrated after dark to keep themselves safe from the Christians.

In the deserts of middle east its too hot during the day

sundown is the beginning of a new day in the Bible

Yeah... that's when most NIGHT CLUBS open....