Vort Written by Rabbi Eliezer Silver on Chiyuv ltahor Bregel - Obligation to Purify Oneself on the Festivals
Below is a partial translation of the Yiddish text
The topic is the obligation to purify oneself before the holiday
There is an obligation to purify oneself on the holiday-which means on the eve of the holiday so that he can be pure at night and make it possible for him to go into the Bais Hamikdash and eat from the sacrifices. And certainly this applies in regard to the holiday of Passover. The Talmud [Gemara] in tractate Rosh Hashanah 16A learns this out from the posuk in Leviticus "and in your carcasses you shall not touch". [e.n. the Talmud there speaks out how we see this from the posuk but it is too lengthy for this discussion]. Also the Safra on page 74 there the rabbis and other people argue. Check the Mishnah Lemelech in the laws of food becoming impure in chapter 16 law 10 that he discusses if this is from the Torah or it is Rabbinically ordained. Even though you do not receive lashes for whatever reason because if you learn from a verse that was expounded then you would not get lashes just like if you have benefit from a live animal. Check there what he says about the words of the Keshef Mishnah and asks on him. He brings in the name of the Ramban that the prohibition is only rabbinically ordained. This prohibition of walking into the temple unpure is not only because you will make holy things impure-we also afraid in Eretz Yisroel about making someone's mundane food impure. [e.n in those days people used to eat even mundane food in purity]. We see from here that a person should be careful not to become impure on the eve of the holiday because maybe he will not find a mikveh to immerse in and then he will be someone who is a tvul yom and he will not be able to eat holy foods at night. The question is nowadays and in the diaspora where we do not bring sacrifices.
[e.n. This is a Mishnah in tractate Meilah chapter 3 Mishnah 5] The milk of consecrated animals and eggs of consecrated turtledoves one may not derive benefit from them but if one does by accident derive benefit from them he is not considered that he took something from the sanctuary [e.n. this is known as meilah]. This prohibition is from the Torah and not Rabbinically ordained because also holy things that are unfit for a sacrifice (i.e. it has a blemish) and you redeemed it, it says in the posuk that you should eat the meat and not the milk. So we see that the prohibition is from the Torah and certainly when it is still holy before it was redeemed because the Mishnah counts it together with the eggs of consecrated turtledoves and there if you wait till the child comes out, the child will have kedushas haguf. Also see the sefer called Efrochei Hekdesh in Talmud Yerushalmi in Tractate Orlah at the end of chapter one and in Tractate Avodah Zora chapter 3 law 5 that Rav Kahane and Rav Yehudah argue there.