They're the same words in the same order — but one has a space and one doesn't, and they mean different things. Here's what you need to know to use everyday and every day correctly.
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Everyday is the adjective, so you'd use it to describe an object or activity that is common: everyday people, everyday hero. Every day is a phrase which describes something which happens each day: every day I write the book, every day I love you more.
In spoken English, no-one is going to notice the difference, but in written English, you absolutely can — yet people getting it wrong remains an everyday occurrence. You should strive every day to avoid that mistake. Accuracy matters.
Lifehacker's Mind Your Language column offers bossy advice on improving your writing.