The text that I will provide below may not be one of "the earliest" texts substantiating that the Jewish Calendar day began at Sunset, but it may be one of the weightiest, at least for Christians.

Now on the first day of the week, Mary Magdalene comes to the tomb early, it being still dark, and she sees the stone having been removed from the tomb. (Jn 20:1, Berean Literal Bible)

John is stating most clearly that "the first day of the week", i.e. Sunday, had already begun while "it being still dark", i.e. before sunrise. Therefore, for John calendar days began at sunset (*).

(*) Unless an argument were made that John used Roman time reckoning, whereby Calendar days began at midnight, which IMV would be preposterous.

The text that I will provide below may not be one of "the earliest" texts substantiating that the Jewish calendar day began at sunset, but it may be one of the weightiest, at least for Christians.

Now on the first day of the week, Mary Magdalene comes to the tomb early, it being still dark, and she sees the stone having been removed from the tomb. (Jn 20:1, Berean Literal Bible)

John is stating most clearly that "the first day of the week", i.e. Sunday, had already begun while "it being still dark", i.e. before sunrise. Therefore, for John calendar days began at sunset (*).

(*) Unless an argument were made that John used Roman time reckoning, whereby for him Calendar days began at midnight, which IMV would be preposterous.