John Calvin Commentary Numbers 29:1

"And in the seventh month, on the first day of the month, ye shall have a holy convocation; ye shall do no servile work: it is a day of blowing of trumpets unto you." — Numbers 29:1 (ASV)

And in the seventh month. I have already observed that the festivals are not (generally) discussed here, but only the sacrifices by which their solemn observance was to be enhanced. In the beginning of the seventh month was the memorial of the blowing of trumpets, as it was called.

Because it was a minor festival, Moses only commands one young bull to be killed; but the number was increased for other reasons, as we have already seen that on the first of every month two young bulls were sacrificed. This day, therefore, had three larger victims, while the number of the others was doubled, so that there were two rams and fourteen lambs.

Thus, God consecrated this day doubly to Himself, so that one celebration did not diminish the other; otherwise, He might have seemed to have repealed what He had once commanded. The memorial of trumpets was not, then, an abolition of the New Moon, but they observed both ordinances at the same time.