“You shall not eat leaven on it” (Devarim 16,3)
It is a negative commandment not to eat any leaven after noon on the fourteenth day of Nissan. About this it says “You shall not eat leaven on it”. The Rambam says that the expression “on it” mentioned in the verse refers to the Passover sacrifice where the obligation to sacrifice it is in the afternoon on the fourteenth of the month and says that from the time appointed to sacrifice it, you shall not eat leaven.
Amongst the roots of the commandment: the topic of forbidding leaven on Passover is an extremely severe prohibition from the point of view of this topic being a big foundation of our faith, for the exodus from Egypt is a proven sign and wonder for the renewal of the world which is the great pillar upon which the halls of Torah lean. Therefore, any commandment which is a reminder of the Exodus from Egypt is far more dear to us and far more severe. Due to its great severity the Torah warns us to begin keeping it six hours before the time affixed to that particular miracle, which is the beginning of the fifteenth day of Nisan. All of this is so that by seeing that the Torah has placed a boundary surrounding the commandment, it will impress upon our hearts the severity of this commandment and the importance of its subject.
This commandment applies to men and to women, in every place and at every time and even nowadays when we do not bring the Passover sacrifice.