The night before Pesah, of course, is when we have to do the Bediqath Hames (the search for the leaven). Before you do the Bediqath Hames you say the Berakha of "Asher Qiddishanu Bemiswothaw Wesiwwanu 'Al Bi'ur Hames." The Berakha is for the burning of the Hames.
Why do we say this Berakha when we are actually only searching for Hames at night, and not burning the Hames until the next day? The answer is that the purpose of checking for the Hames at night is in order to burn it the next day. That is why say the berakha at night. And even though it is a Miswa that comes every year at this specific time, we do not say the Berakha of Sheheheyanu.
However, as it says in Ben Ish Hai, if a person can find a new fruit or an important new item of clothing on which you would normally say the Sheheheyanu, then he should say the Sheheheyanu on that item and have in mind the Bediqah (the search for leaven) also.
(See Ben Ish Hai, Shana Rishonah, Parashath Saw, Oth Heh)
