On Shabbos we say, mekadesh hashabbos. On Yomtov we say, mekadesh yisrael vehazemanim. Grammatically, Shabbos is singular; zemanim is plural. On Rosh Hashana and Yom Kippur we also use the singular. We say, mekadesh yisrael veyom hazikaron and mekadesh yisrael veyom hakippurim, respectively.The explanation of the two last seems pretty straightforward. Rosh Hashana and Yom Kippur both occur once through the span of the year. Hence the singular rendering. A corresponding explanation is at hand for Yomtov. Yomtov occurs multiple times during the year: there is Pesach Shavuos, Sukkos, and Shemini Atzeres. Hence the plural inflection of the noun zemanim. But this raises a question about the singular inflection of the noun Shabbos. Shabbos, too, occurs multiple times through the course of the year. Why, then, does it not receive the plural form in the beracha – which is recited in the Shemoneh Esrei and in Kiddush?
Ostensibly, we can answer as follows. Yes, Shabbos occurs multiple times; but each occurrence is an occurrence of the same day, Shabbos. And it has the same kedusha, the kedusha of Shabbos. In other words, we invoke the traditional genre-species/type-token distinction. Each occurrence of Shabbos is of the same kind, even though it is a numerically discrete instance of this kind.
We can go on to explain the difference between the beracha for Shabbos and the beracha for Yomtov in these terms. With Shabbos, there is one entity – kind, type, etc – of Shabbos that occurs repeatedly throughout the year. With Yomtov, on the other hand, there are multiple such entities (types, kinds, or whatever you want to call them). Pesach is its own entity, Shavuos is its own, and so on. Seemingly, each has its own type of kedusha. That of Pesach is not the same as that of Shavuos, and so on. (One may raise the question, though: Why are they lumped together in one nusach haberacha? Why isn’t there a separate beracha that reads mekadesh yisrael vehapesach, and similarly for the others?) Accordingly, we use the plural inflection of mekadesh yisrael vehazemanim.A question that, however, remains pertains to Rosh Chodesh. We use the plural form: mekadesh yisrael veroshei chadashim. Arguably, Rosh Chodesh is repeated twelve (or thirteen) times during the year. It is the same sort of entity each time – just as one and same entity of Shabbos is unitary, as it is repeated throughout the course of the year. Why, then, does the beracha of Rosh Chodesh not follow the model of Shabbos and take the singular form? Why do we say, roshei chadashim rather than rosh chodesh?
Perhaps we should draw the conclusion that each month, and therefore each Rosh Chodesh, is an individual entity, set apart from all the other months. One Rosh Chodesh is not a repeat of the preceding one or of the next. It is unique: an entity (or type) unto itself – Nissan, Iyar, Sivan, etc.If so, why? Let me leave it to you for your contemplation.