Marie,

I read it that, 'which are not fit to be done on common days"?' refers to the sins which men do.
In other words it might read,"How provoking must it be to God, when men do on that day (which he has sanctified, and set apart to be spent in the immediate exercises of religion) those things which are not fit to be done on common days, which are impure and wicked whenever they are done!"
Hope that's some help.
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