The Shulchan Aruch writes (469:1; listen to audio for precise citation) that one may not say regarding a live animal or piece of meat, "This is for Pesach." This refers not to saying, "This is for the Korban Pesach [Paschal offering]," which is certainly forbidden, but rather to verbally designating meat for the festival of Pesach. Halacha forbids doing so because it appears as though one designates the meat for the Korban Pesach, thus giving the impression that he plans on partaking of sacrificial meat outside of Jerusalem, in violation of the Torah. If one wishes, he may verbally designate the meat "for Yom Tov," without mentioning specifically the festival of Pesach.
Some authorities claim that this prohibition applies as well to making such a statement in writing. Meaning, stores advertising their Pesach selection should not post signs that say, "Meat for Pesach." The signs should instead read, "Meat for Yom Tov."
Furthermore, according to some authorities this prohibition applies not only to meat, but also to fish and poultry. When it comes to wheat, however, one may say, "This is for Pesach," because it is clear that he intends merely to protect the wheat from Chametz so he can use it on Pesach, and he does not refer to a sacrifice.
Summary: One may not declare verbally or in writing about a portion of meat, poultry or fish, "This is for Pesach"; he may say or write about the food, "This is for Yom Tov."
