Why, in middle blessing of the amida for Shabbat is the blessing's conclusion in the singular (mekadesh hashabbat) while for holidays and Rosh Chodesh, the wording is in the plural (z'manim and roshei chodashim)?
I know that the verses lead in this direction for shabbat and R"C (וביום השבת vs. ובראשי חדשיכם) but isn't the b'racha about sanctifying either this specific day as an example of something, so it should be for all, in the singular, or all the days, so it should be consistently in the plural?
As a side note, towards the beginning of the middle blessing of Shabbat that coincides with R"C or holidays, we mention Shabbat in the plural (Shabbatot limnucha) and then both in the singular (v'et yom...) but close the bracha in the mix.