Before Pesach, a person is obligated to perform bedikas chometz, a search of his house and possessions, to ensure that he does not own any chometz. The bedika should be conducted at the beginning of the night of the 14th of Nissan, immediately after tzeis hakochavim.
1. M.B. 431:1. Also see Halichos Shlomo (Pesach 5:10) quoting Rav Shlomo Zalman Auerbach, zt”l.