The sages decreed that the search for chametz should take place on the night of the fourteenth of Nissan. This time was chosen because people are usually at home in the evening, and because one cannot search effectively in the corners by daylight. It is necessary to use a candle for this, and since candlelight has little effect during the day, it was decreed that the search be done in the evening.
The correct time for the search is when the stars appear. Although it is common practice to perform a mitzvah at the earliest possible time, this mitzvah should not be performed before the appearance of the stars. The search should be started immediately when the stars appear, while there is still a bit of daylight.
From half an hour before the stars appear, at the time when the sun sets on the thirteenth of Nissan, one should be careful not to begin any work, nor should one eat, until after he has searched for the chametz. Even if one usually engages in Torah study at this time, he should postpone it until after the search. If one began to study or to work at a time while it is still permissible, he should interrupt whatever he is doing as soon as the time for the search arrives.
Those who will be praying the evening prayers with a quorum should pray before the search, for it is possible that they will be unable to find a quorum of ten men later. Those who usually pray alone should search first, for we have no reason to fear that they will forget to pray the evening prayers later.
If one did not search for chametz on the eve of the fourteenth, he should do so during the day itself ? by candlelight rather than by daylight. If the natural light is very strong, for example, in a hall which is completely open on one side, one may conduct the search by daylight. This is also true of rooms and houses which have many windows and much light. The area near the windows may be checked by using daylight, provided that the windows are open.
If one failed to search for chametz by the time the chametz becomes prohibited, i.e., by midday on the fourteenth of Nissan then he should conduct the search afterwards. If he did not do so then, he should conduct the search on Passover itself, and if he discovers chametz, he should destroy it immediately. If he failed to conduct the search either before or during Passover he should do so after Passover.
He must burn any chametz which remained in his house from before Passover, or dispose of it in a way that ensures that no one can derive any benefit from it.
If a person is about to set off on a prolonged trip and has no intention of returning home until after Passover, and there is no one in his home who can conduct a search for chametz at the designated time, he must perform the search by candlelight on the evening prior to his departure. However, this only applies if he leaves his home up to thirty days before Passover. If he leaves his home before then, he does not have to conduct a search for chametz, but if he begins his trip more than thirty days before Passover and intends to return home for the Festival, he must undertake the search before leaving, for it is possible that he may only return at the last moment and will not have time to dispose of the chametz then.
If a person leaves on a trip more than thirty days before Passover, and if he has no intention of returning before the Festival, he need not search for chametz. However, some authorities maintain that if he knows there is chametz in his house, he must dispose of it even if he has no intention of returning to his house before Passover.
If a person moves out of his home less than thirty days before Passover, he must dispose of the chametz unless he knows that another Jew, who will be careful to dispose of the chametz, will move into the house during the period before Passover.
Why was the time limit set at thirty days? Our Sages ruled that thirty days before the Festival, one should study and discuss the laws of Passover. Thus, the obligation to search applies from that time.
If the day preceding Passover falls on Shabbat, the search is undertaken on the night of the thirteenth of Nissan, i.e., on the previous Thursday evening, and sufficient chametz food is set aside for the Shabbat meals.