Everyday, every day, day by day confused!

Dear Khalid
Everyday means ordinary, common or regular.
    In his everyday life he was a mechanic, but he dreamed of becoming a film star.
Every day means all the time or each day.
    The museum is open every day except Friday.
Day by day means one day after another like day after day. The difference is that day by day is often positive (things getting better) while day after day is often negative. I guess you are a native-Arabic speaker, so these two have a different meaning in English - not every other day (day, miss one, day) - but every day (day, day, day).
    The baby is getting bigger day by day. (He is getting bigger today, tomorrow, and the next day.)
    Although he tried day after day, he never found a job. (He tried today, yesterday, and the day before.)

I hope this helps!
Teacher Jill