Concise Bible Dictionary:

The period from sunset till night. This was naturally the closing of the day, for God called the light “day” (compare John 11:99Jesus answered, Are there not twelve hours in the day? If any man walk in the day, he stumbleth not, because he seeth the light of this world. (John 11:9)). “The evening was, and the morning was, one day”; that is, there was not day continuously, but through the alternation of night and morning day succeeded day. Gen. 1:55And God called the light Day, and the darkness he called Night. And the evening and the morning were the first day. (Genesis 1:5). The common way of reckoning the day among the Jews was from evening until the next evening. A difficulty has arisen as to the phrase “between the two evenings.” The paschal lamb was to be killed between the two evenings, and some have thought that this allowed the evening of the 15th Abib. This however cannot be the meaning because none of it was to be left till the morning; and because the same phrase is used respecting the daily sacrifice, and also as to lighting the lamps (Ex. 12:66And ye shall keep it up until the fourteenth day of the same month: and the whole assembly of the congregation of Israel shall kill it in the evening. (Exodus 12:6), margin; Ex. 29:39; 30:839The one lamb thou shalt offer in the morning; and the other lamb thou shalt offer at even: (Exodus 29:39)
8And when Aaron lighteth the lamps at even, he shall burn incense upon it, a perpetual incense before the Lord throughout your generations. (Exodus 30:8)
). The Jewish writers are not agreed in their definition of the expression: some suppose it lies between the beginning and ending of sunset; others, from sunset to full darkness. Josephus says that the time of killing the passover was from the ninth hour till the eleventh, which would be about from three o’clock to five; but this would seem to make the “evening” come at the end of the Jewish day, and not at the beginning.