לפני ה' אלהיך תאכלנו. לַכֹּהֵן הוּא אוֹמֵר, שֶׁכְּבָר מָצִינוּ שֶׁהוּא מִמַּתְּנוֹת כְּהֻנָּה אֶחָד תָּם וְאֶחָד בַּעַל מוּם, שֶׁנֶּאֱמַר (במדבר י"ח) וּבְשָׂרָם יִהְיֶה לָּךְ וְגוֹ' (בכ' כ"ח): לפני ה׳ אלהיך תאכלנה THOU SHALT EAT IT BEFORE THE LORD THY GOD — Scripture is speaking to the priest (not to the owner to whom תקדיש in the previous verse refers), for we find already stated that it (the בכור) is one of the dues of the priests whether it is unblemished (and its blood and fat portions are to be sacrificed on the altar and the flesh eaten by the priests), or whether it is blemished (when it must not be offered on the altar), for it states, (Numbers 18:18) “and the flesh of them (the firstborn animals, both the blemished and the unblemished) shall be thine” (the priest’s) (cf. Bekhorot 28a).
שנה בשנה. מִכָּאן שֶׁאֵין מַשְׁהִין אוֹתוֹ יוֹתֵר עַל שְׁנָתוֹ; יָכוֹל יְהֵא פָּסוּל מִשֶּׁעָבְרָה שְׁנָתוֹ, כְּבָר הֻקַּשׁ לְמַעֲשֵׂר, שֶׁנֶּאֱמַר (דברים י"ד) וְאָכַלְתָּ לִפְנֵי ה' אֱלֹהֶיךָ מַעְשַׂר דְּגָנְךָ תִּירֹשְׁךָ וְיִצְהָרֶךָ וּבְכֹרֹת בְּקָרְךָ וְצֹאנֶךָ, מַה מַּעֲשֵׂר שֵׁנִי אֵינוֹ נִפְסָל מִשָּׁנָה לַחֲבֶרְתָּהּ אַף בְּכוֹר אֵינוֹ נִפְסָל, אֶלָּא שֶׁמִּצְוָה תּוֹךְ שְׁנָתוֹ (בכ' כ"ז): שנה בשנה [THOU SHALT EAT IT BEFORE THE LORD …] YEAR BY YEAR — From here we derive the law that one should not defer it (i.e. sacrificing and eating it) beyond its first year (Bekhorot 28a). If so, one might think that it becomes invalid as a sacrifice as soon as its first year has passed! This is not the case, for it has been put in comparison with the second tithe, as it is said, (Deuteronomy 14:23; cf. Rashi on that verse): “and thou shalt eat before the Lord thy God the tithe of thy corn, of thy wine, and of thine oil, and the firstlings of thy herds, and thy flocks”. How is it in the case of the second tithe? It does not become invalid if left over from one year to the other (cf. Rashi on Deuteronomy 14:28 and 29), so, too, does the firstborn animal not become invalid under the same circumstances, only that it is a meritorious act to offer and consume it within its first year.
שנה בשנה. אִם שְׁחָטוֹ בְּסוֹף שְׁנָתוֹ אוֹכְלוֹ אוֹתוֹ הַיּוֹם וְיוֹם אֶחָד מִשָּׁנָה אַחֶרֶת, לִמֵּד שֶׁנֶּאֱכָל לִשְׁנֵי יָמִים וְלַיְלָה אֶחָד (שם): שנה בשנה (may also be translated A YEAR WITH ANOTHER YEAR) — If one slaughtered it at the end of its first year (an the last day) he may eat it that day and the first day of the next year. This teaches us that it (a firstborn animal) may in all cases be eaten on two successive days and the intervening night (cf. Sifrei Devarim 125; Bekhorot 27b).