The Torah obligation to eliminate one’s Hametz on Ereb Pesah can be fulfilled in one of two ways – through either "Bittul," renouncing one’s ownership over his Hametz, or "Bedikat Hametz," thoroughly searching one’s home for Hametz and eliminating all the Hametz he finds. However, the Sages enacted that even if one makes a "Bittul" declaration, he should nevertheless perform "Bedilka" and eliminate the Hametz from his property. One reason is that the Sages were concerned that one might not proclaim the "Bittul" wholeheartedly, and will not fully intend to renounce his ownership over his Hametz, in which case it is invalid and the Misva is not fulfilled. The second explanation is that since we normally eat Hametz all year round, we might mistakenly eat Hametz that we find in our home during Pesach. To avoid this risk, the Sages enacted that besides the verbal proclamation of "Bittul," one should also search for all the Hametz in his home and eliminate it.
The Mishna in the beginning of Masechet Pesahim teaches that one performs the search for Hametz "Or Le’arba’a Asar," which the Gemara explains to mean on the night of the 14th of Nissan – meaning, the night before Pesach. The Shulhan Aruch (Orah Haim 431) writes that the "Bedika" should be done "Bi’thilat Ha’layla" – "at the beginning of the night." Hacham Bension Abba Shaul (Israel, 1924-1998) explains that although the Misva can be fulfilled throughout the night, one should perform the search at the beginning of the night, at Set Ha’kochavim (when the first stars can be seen), in accordance with the rule of "Zerizin Makdimin Le’misvot" (one should perform a Misva at the earliest opportunity). However, Hacham Bension adds, when the Sages said that the "Bedika" should preferably be done right at the time of Set Ha’kochavim, they took into consideration the recitation of Arbit. Following the rule of "Tadir Ve’she’eno Tadir, Tadir Kodem" – that a more frequent Misva should precede a less frequent one – one should first pray Arbit at Set Ha’kochavim – some 20 minutes after sunset – and then immediately proceed to the "Bedika."
The Hafetz Haim (Rav Yisrael Meir Kagan of Radin, 1839-1933) writes that once the time of Bedikat Hametz arrives, one should not eat even a small portion of food, such as a fruit, until after fulfilling the Misva.
As "Bedikat Hametz" is performed before one makes the "Bittul" proclamation, the "Bedika" has the status of a Misva De’Orayta – the fulfillment of a Biblical Misva. It is thus a very precious opportunity, and one should not at all minimize the great value and importance of the search for Hametz on the night before Pesach.
Summary: On the night before Pesach, one should pray Arbit right at the time of Set Ha’kochavim (approximately 20 minutes after sundown) and then immediately proceed to perform "Bedikat Hametz." One should not eat even a small snack from the time of Set Ha’kochavim until after completing the search for Hametz.
