Edersheim's "The Temple" (1969 edition) in chapter 11 The Passover page 211-212 concludes "from the contemporary testimony of Josephus (Jew. Wars, vi. 9, 3) and from Talmudical authorities, there cannot be a doubt that, at the time of our Lord, it was regarded as the interval between the sun's commencing to decline and his actual disappearance."
That was about the half-way point from high noon until actual sunset.
The Smararitans, the Karaite Jews, and many modern interpreters regarded evening as between actual sunset and nightfall/complete darkness, 6P-7P varied by seasons (page 212).
There's some opinion, his included, the first evening was based on the timing of the Passover sacrifice which began then at 2:30 (9th hour watch beginning at 2), when a lamb should be killed, the second evening at 3:30 at Jerusalem when it is offered, concerning provision of making adequate time to prepare for then observe everything else required by sunset (such as prayers), else activities fell into the next day, so evening was backed up to the afternoon. Low lying towns would experience the end of day earlier, so there was about an hour difference across Israel because of location, high or low lying. If the sun couldn't be seen, then they went by when the fowls went to roost. They don't do that right at full darkness, another little detail against the Samaritan opinion.