Every day or everyday: which is correct?

Correct
every day (adverb: each day) / everyday (adjective: ordinary)
Avoid
everyday (for "each day": "I run everyday")
Avoid
every day (before a noun: "every day shoes")
Summary

Correct: every day (adverb: each day) / everyday (adjective: ordinary) — Avoid: everyday (for "each day": "I run everyday") / every day (before a noun: "every day shoes"). Every day = on each day, used as an adverb; everyday = ordinary, commonplace, used only as an adjective before a noun.

Part of speech
adverb / adjective
Source
Merriam-Webster

Why

Merriam-Webster: everyday (one word) is an adjective meaning ordinary or daily, as in everyday life; the two-word every day is an adverbial phrase meaning each day. If you can insert "single" (every single day), it is two words.

Examples

  • I check my email every day.
  • These are my everyday shoes.
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Checked against
Merriam-Webster / Oxford
Spelling authority
Merriam-Webster · Oxford English Dictionary / Oxford spelling · SCOWL
Commonly confused and non-standard forms
64
Dictionary
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