Luke 1:59

‘And it came about on the eighth day, that they came to circumcise the child; and they would have called him Zacharias, after the name of the father, and his mother answered and said, “Not so, but he shall be called John.” ’

And on the important eighth day after the birth they all gathered together to celebrate the circumcision of the child. Every son of Israel was circumcised on the eighth day (otherwise he was seen as cut off from among the people). If necessary it could even be done by a woman or on the Sabbath. It may be that on this day he was officially named, although of course the baby would already have had a name applied to him. Previously naming was always done at birth. This is the first evidence for what would much later become the practise (compare also Luke 2:21).

On the other hand it may simply be that due to the unusual circumstances of the father being dumb and the mother in retirement the name had been given to the baby at birth but had not become generally known, so that they just assumed that nothing had been done and decided that he must be called Zacharias on the assumption that that was what the parents would expect to call him, after his father. This was the custom in certain families. ‘Not so,’ Elisabeth said (obediently to God’s command - Luke 1:13), ‘he shall be called John’. Her husband had clearly communicated some of his experience to her in writing and she was determined to stick with the name already given to him.