Not to use the vessel for Chametz 24 hours before Kashering:[1]

The custom is to only do Hagala to vessels which are not Ben Yomo, which means that they have remained without use of hot Chametz, or soaking Chametz, for 24 hours. When Kashering on Erev Pesach past the 5^th hour of the day, then from the letter of the law the vessels may not be Ben Yomo, unless one has a total of 60x in the Hagala water versus the total amount of vessels being Kashered. [Some Poskim[2] write that initially the vessel is to wait 24 hours from after it is cleaned. Other Poskim[3] rule this is not necessary.]

[1] Admur 452:9-15; Rama 452:2

Background: From the letter of the law, the Chametz vessels being Kashered before the 5^th hour on Erev Pesach may be Ben Yomo and even in such a case it is not necessary for there to be 60x amount of water in the Hagala vessel versus the vessel being Kashered. The reason for this is because Chametz before the 5th hour is still permitted, and thus it has the status of Nat Bar Nat Dihetera. [Admur 452:13] However, the custom is to that all vessels being Kashered are to be not Ben Yomo, even when Kashering before the 5^th hour on Erev Pesach and even if there is 60x the amount of water against all the vessels. The reason for this stringency is because we suspect that perhaps one of the vessels may have absorbed Issur, and is Ben Yomo, in which case if there is not 60x in the water then it forbids all the other pots, as it is a case of Nat Bar Nat Dissurah. [Admur 452:15] If, however, one is doing Hagala after the 5^th hour on Erev Pesach, then the vessels may not be Ben Yomo unless there is a total of 60x in the water. [Admur 452:9-10]

Kashering meat and milk vessels: Even before the 5^th hour, if the vessels are Ben Yomo and there is not 60x in the water against the total amount of vessels being Kashered, then one must be careful to not kasher milk and meat vessels simultaneously. However, they may be placed one after the other, as its Nat Bar Nat Dihetera. [Admur 452:14]

[2] M”B 452:20

[3] Implication of Admur ibid; Chok Yaakov, brought in Shaar Hatziyon 452:25