Boyd’s Bible Dictionary:

(decline of day). Two evenings recognized, one before, the other after, sunset (Gen. 24:63; Ex. 12:6; Num. 9:3; 28:4).

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 11:9). “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. 1:5. 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. 12:6, margin; Ex. 29:39; 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.