1. On the night of the fourteenth, after conducting the search for chametz, one must nullify any chametzthat is left in his possession that he did not see and is unaware of. [However, one does not nullify chametz that he is aware of and intends to eat or burn in the morning] (Shulchan Aruch and Mishnah Berurah 434).
2. It is proper to nullify one’s chametzagain in the morning of the fourteenth, immediately after burning the chametz and before halachic midday. At that point, he should nullify both the chametz that he is aware of and that he is not aware of. [This must be done before midday because after midday the chametz is halachically no longer in his possession.] (ibid.).
