The Shulchan Aruch[2] writes: “It is good to eliminate [chametz] on Erev Shabbat before midday [apparently, an hour before midday][3] so that people do not err in other years and eliminate the chametz after midday.” Despite this, one may leave the amount of chametz that he plans to eat through the (early) Shabbat morning meal.[4] This halachic suggestion is not an absolute halacha and should not create a situation whereby, due to haste, one might miss some chametz and end up never disposing of it.
The question of mechirat chametz is more complicated, and there are different possibilities and practices. The individual should follow the system the local rabbinate is using. However, it is worthwhile to be aware of the issues, not only for the sake of understanding Torah but also to act appropriately in accordance with the particulars of the local practice.
Acharonim discuss whether the desire to follow the schedule of regular years applies to mechirat chametz as well. Is selling the chametz an accepted method of eliminating it, in which case it should be done by Friday morning, or is it a way of using the chametz,[5] which one may continue until the deadline on Shabbat morning?[6] According to the stringent approach, the rabbi should carry out the sale to a non-Jew on behalf of his congregation during the latter part of Friday morning. However, stringency sometimes causes more halachic problems than it solves. Those who continue to deal with their chametz during the afternoon (e.g., storeowners) and those who add chametz that they previously had not expected to sell to that which was sold will unknowingly have missed the sale for those items. Therefore, most rabbanim will not rely on only an early sale, and if they want to be machmir to perform mechirat chametz at the earlier time, will perform an additional, later sale to cover last minute issues that the first one did not include.
There is a further question as to how late one may execute the sale. Some say that when doing the sale on Friday, the rav can stipulate with the seller, orally and in the document, that the sale will take effect soon before the time that it becomes forbidden to benefit from chametz – on Shabbat. This can conceivably be done in one of two ways: The effective time of the sale can be delayed to Shabbat morning, or, alternatively, the sale can possibly take effect at the time of the transaction on Erev Shabbat, but with the items to be included retroactively determined by what remains in the seller’s possession on Shabbat. Either way, one cannot include chametz that he did not own when the acts of transaction took place. However, having the sale take effect on Shabbat is controversial, even if one is inactive on Shabbat. Rav Akiva Eiger[7] and others say it is a violation of the prohibition of commerce on Shabbat.
Rabbanim who do not want the sale to be completed on Shabbat can have it take effect shortly before Shabbat. This can also be done in one of two ways: It can be done on a time delay, in which case the rabbi can pick a time that is very close to Shabbat and delay the effective time of the sale to then. Alternatively, f he is concerned that a time delay might not be not appropriate for this type of sale, he can actually do the transaction as close as possible to Shabbat. In these cases, the owner of the chametz must be aware of the time that the transaction will take place, and decide by then what to keep for Shabbat and what to include in the sale; once the sale takes effect, he cannot include any other leftover chametz therein.
Let us reiterate that the local rabbinate will choose, based on halachic and/or practical considerations, which system to use. The individual should make sure he understands how he is expected to comply with it.