The folks were fasting but God was not impressed.  Why not?  They really were making themselves hungry, really were going without for a time.  But God belittled their efforts.  In effect, He said, “You call that a fast?  Are you kidding?  That’s not fasting, not even close.”   Check it out for yourself: it’s in the 58th chapter of Isaiah.  But here is the nub of it:

⁵ Is this the kind of fast I have chosen,
only a day for people to humble themselves?
Is it only for bowing one’s head like a reed
and for lying in sackcloth and ashes?
Is that what you call a fast,
a day acceptable to the Lord?
⁶ “Is not this the kind of fasting I have chosen:
to loose the chains of injustice
and untie the cords of the yoke,
to set the oppressed free
and break every yoke?
⁷ Is it not to share your food with the hungry
and to provide the poor wanderer with shelter—
when you see the naked, to clothe them,
and not to turn away from your own flesh and blood?  (Isaiah 58:5-7)