I saw this on an Ikea sign.

Problem:
“Everyday” is an adjective, but an adverb is required here.

Explanation:
“Everyday” tells you what. For example, an “everyday event” is an event that occurs daily. In other words, “everyday” is an adjective; it modifies a noun (such as “event”). “Everyday” in this example tells you what kind of event it is. In contrast, “every day” tells you when. For example, “It happens every day.” The goal of the sign maker is to tell the reader when the store is open.

Solution:
“Every Day – 10 AM to 9 PM”