What's the difference between "o zi a săptămânii" and "o zi din săptămână"? I found the second one in another sentence
saptamanii - seven tomorrows?
No, săptămână comes from Latin septimana (source of Italian settimana, French semaine, Spanish semana, etc.), feminine form of septimanus, "related to the seventh element of a series".
Technically? I'm sure it's what JCMH said. If it helps you remember it better, then heck yes it's seven tomorrows. :D