Jamieson, Fausset and Brown on Matthew 12:5

5. Or have ye not read in the law, how that on the sabbath days the priests in the temple profane the sabbath--by doing "servile work."

and are blameless?--The double offerings required on the sabbath day (Nu 28:9) could not be presented, and the new-baked showbread (Le 24:5; 1Ch 9:32) could not be prepared and presented every sabbath morning, without a good deal of servile work on the part of the priests; not to speak of circumcision, which, when the child's eighth day happened to fall on a sabbath, had to be performed by the priests on that day. (See on Joh 7:22, 23).