Every day is a noun phrase and can be used as an adverbial. For example:
We go to school every day. We go to school every day.
Mathematics is a part of daily life. Mathematics is a part of daily life.
The girl helps her mother wash the dishes after supper every day.
I like it because I look at it every day and think about everyone in it! I like it very much, because every day when I see these photos, I think of all the people in them.
Sheldon writes every day and dictates his novels in the morning. Sheldon writes seven days a week, dictating novels to others every morning.
He licks his face every day. He washes his face carelessly every day.