As an editor, I see these words confused almost every day. One might say it’s an everyday occurrence.

Everyday with no space means “ordinary” or “typical,” as in Tom wore his everyday clothes to work on Fridays. In this case, the clothes Tom wore are what he usually dons at home.

Every day with a space means “each day,” such as Mrs. Crabapple takes attendance every day before beginning her lesson.

An easy way to remember the difference is that the every day meaning each day is two words, just like its definition. You wouldn’t write eachday.