The 4th of Ab, 455 B.C.E.
In Nisan (March-April) of the 20th year of Artaxerxes’ rule (455 B.C.E.), Nehemiah petitioned the king: “If your servant seems good before you, . . . send me to Judah, to the city of the burial places of my forefathers, that I may rebuild it.” (Ne 2:1, 5) The king granted permission, and Nehemiah made the long journey from Shushan to Jerusalem. On about the fourth of Ab (July-August), after making a night inspection of the walls, Nehemiah gave the command to the Jews: “Come and let us rebuild the wall of Jerusalem, that we may no longer continue to be a reproach.” (Ne 2:11-18) Thus, “the going forth of the word” to rebuild Jerusalem, as authorized by Artaxerxes, was put into effect by Nehemiah in Jerusalem that same year. This clearly establishes 455 B.C.E. as the year from which the 70 weeks would begin to count. it-2 p. 900
Interestingly, earlier in this same Insight volume, it mentions that the 20th year may not have started until Tishri (September/October) by Nehemiah's reckoning:
The month of Chislev (November-December) of a certain 20th year is the reference point with which the historical narrative begins. (Ne 1:1) As is evident from Nehemiah 2:1, this 20th year must be that of Artaxerxes’ reign. Obviously, the 20th year in this case is not reckoned as starting in Nisan (March-April), for Chislev of the 20th year could not then precede Nisan (mentioned at Ne 2:1) of the same 20th year. So it may be that Nehemiah used his own count of time, reckoning the lunar year as beginning with Tishri (September-October), which month Jews today recognize as the beginning of their civil year. Another possibility is that the reign of the king was reckoned from the actual date that the monarch ascended the throne. This could be so even though the Babylonian scribes continued to reckon the years of the Persian king’s reign on their customary basis of a Nisan-to-Nisan count, as their cuneiform tablets show they did. it-2 p. 487-488
But, to answer your question, Ab 4, 455 B.C.E.