2 Kings 25:8

8 And in the fifth month, on the seventh day of the month, which is the nineteenth year of king Nebuchadnezzar king of Babylon, came Nebuzaradan, captain of the guard, a servant of the king of Babylon, unto Jerusalem:

Jeremiah 52:12

12 Now in the fifth month, in the tenth day of the month, which was the nineteenth year of Nebuchadrezzar king of Babylon, came Nebuzaradan, captain of the guard, which served the king of Babylon, into Jerusalem,

Was it the 7th day of the 5th month, or the 10th day of the 5th month?

Notice in 2 Kings, we are being told when Nebuzaradan came "unto" Jerusalem. Three days later, we learn from Jeremiah, he came "into" the city.

Why did he take three days to get into the city? Because his intentions were to sack it (2 Kings 25:9-11, Jeremiah 52:13-15). The remnant of people in the city were able to hold him off for three days.