It is called this because Nisan 15, which is the first day of the seven - day Festival of Unfermented Cakes (and is always a Sabbath, no matter on what day of the week it comes), falls on the same day as the regular Sabbath.
Seda kutsutakse niimoodi sellepärast, et 15. niisan, mis on seitsmepäevase hapnemata leibade püha esimene päev (ja alati hingamispäev, vaatamata sellele, millisele nädalapäevale ta langeb), langeb kokku tavalise hingamispäevaga.