John Gill Commentary Numbers 29:12

"And on the fifteenth day of the seventh month ye shall have a holy convocation; ye shall do no servile work, and ye shall keep a feast unto Jehovah seven days:" — Numbers 29:12 (ASV)

And on the fifteenth day of the seventh month you shall have
an holy convocation
Of the same month Tisri, which was the seventh from Nisan or Abib, though it was formerly the first month of the year:

you shall do no servile work, and you shall keep a feast unto the Lord
seven days ;
the feast of tabernacles, which began on the fifteenth day of this month.