We learned in the first Mishna in Massekhet Zevaḥim (2a) that if the korban Pesaḥ – the Passover sacrifice – were brought at the proper time – the 14th day of Nissan – with improper intent, it will not be acceptable. The Gemara on today’s daf quotes a baraita that clarifies this matter further. According to the baraita, a korban Pesaḥ that is brought at the proper time with proper intent will be valid, but if it is brought with improper intent it will not be acceptable. If the korban Pesaḥ was brought on a different day of the year, however, it will be an acceptable sacrifice if it was done without intending it as a korban Pesaḥ. If the person meant to bring it as the korban Pesaḥ, then it would be invalid as a sacrifice.
The Gemara continues and explains that since the korban Pesaḥ can only be brought on a specific day of the year, if an animal that was consecrated for that sacrifice was brought on a different day, and the owner did not intend for it to be a korban Pesaḥ, then it is acceptable as a korban shelamim – a peace-offering (see above, daf 4).
In the continuation of the Gemara we find different passages that are brought as sources for the idea that the korban Pesaḥ can be brought as a korban shelamim if it is sacrificed on a different day during the year without the intention that it should be a korban Pesaḥ. At no point, however, does the Gemara offer a source for the fact that if the person’s intention was that it should be a korban Pesaḥ it is invalid. Rashi suggests that since the date of the Passover sacrifice – the afternoon of the 14th of Nissan – is repeated several times in the Torah, it should be understood to mean that any other day is invalid. Tosafot brings a slightly different approach in the name of the kuntros (which usually refers to Rashi), based on logic rather than on the Biblical passages. He argues that it is clear that the only possible date when the korban Pesaḥ can be brought in a meaningful way is on the holiday of Pesaḥ, and not on any other day of the year.