This word and phrase look very similar and sound almost identical. Therefore, in conversation, there is little chance of making an error, but when writing, be sure to think carefully about which word you choose.
Every day - here every is a determiner, and day is a noun. When you say “every day”, you mean each day without exception.
For example: You have been late for school every day this week. Every day, I run 5 kilometers.
Everyday is an adjective. When you say “everyday”, you mean ordinary, unremarkable, usual.
For example: My culture pages offer an insight into the everyday life of Britain. I am going shopping to buy some new everyday clothes.
Note: if you want to check yourself, see if you could write “every morning” or “every evening” without loss of meaning. Then you’ll know it should be “every day”.