Section 21(2)(a) in The Representation Of The People Act, 1950
(a)shall, unless otherwise directed by the Election Commission for reasons to be recorded in writing, be revised in the prescribed manner by reference to the qualifying date—(i)before each general election to the House of the People or to the Legislative Assembly of a State; and(ii)before each bye-election to fill a casual vacancy in a seat allotted to the constituency; and