One person might believe that one day is more important than another. And another person might believe that every day is the same. Each person should be sure about his own beliefs in his own mind.
The person that thinks one day is more important than other days is doing that for the Lord. And the person that eats all kinds of food is doing that for the Lord. Yes, he gives thanks to God {for that food}. And the person that refuses to eat some foods does that for the Lord. And he gives thanks to God.