John Gill Commentary Numbers 29:35

"On the eighth day ye shall have a solemn assembly: ye shall do no servile work;" — Numbers 29:35 (ASV)

On the eighth day you shall have a solemn assembly
The day after the seven days of the feast of tabernacles were ended; for this was not properly a part of that feast, but was a sort of appendage to it;

you shall do no servile work therein ;
(See Gill on Leviticus 23:36).