1) Some have the custom not to say Mizmor Lessodah on the eve of Passover and the intermediate days of Passover, as on these days one may not bring the Thanks Offering, as there leavened loaves brought with it. The same applies to the eve of the day of atonement as one may not offer the thanksgiving offering on that day, as by doing so one reduces the amount of time available for eating (as the next day one may not eat) Rema 51
2) There are, however, others who contend that even on these days one does recite the aforementioned psalm, as it is just thanking Hashem and not replacing the offering. (Beis Yosef and Kaf Hachaim)
