One person considers one day to be more holy than another. Another person thinks all days are the same. Each of them should be absolutely sure in their own mind.
I am absolutely sure that nothing is “unclean” in itself. The Lord Jesus has convinced me of this. But someone may consider a thing to be “unclean.” If they do, it is “unclean” for them.
But whoever has doubts about what they eat is guilty if they eat. That’s because their eating is not based on faith. Everything that is not based on faith is sin.
But not everyone knows this. Some people still think that statues of gods are real gods. They might eat food sacrificed to statues of gods. When they do, they think of it as food sacrificed to real gods. And because those people have a weak sense of what is right and wrong, they feel guilty.