Getting back to the topic, does the shiur of learning hilchos Yom Tov 30 days before the chag contradict the shiur of learning hilchos chag the day of the chag? Most of the comments to the previous post intutited the answer my son said over at his siyum, but which interestingly is not found in the classical earlier achronim - I guess we have all been brainwashed by lomdus to think more immediatly in these terms.
The Ramban writes that the pasuk “VaYidaber Moshe es moadai Hashem el Bnei Yisrael” refers to the takanos of learning hilchos Yom Tov on Yom Tov and reading the Torah parsha of the day, citing the last gemara in Mes. Megillah. The Ramban continues that this same idea is conveyed in Onkelus’s explanation, which he quotes - Onkelus writes that Moshe taught the Jewish people the calculations of the calendar that fix the precise day when each holiday will occur. Isn't this Ramban self-contradictory? What do the instructions of how to calculate a calendar have to do with the takanah of learning hilchos Yom Tov? How can these both be called the same interpretation?
Rav Soloveitchik (in Shiurim l'Zecher Aba Mari) explained that both interpretations convey the same fundamental point. “VaYidaber Moshe es moadei Hashem” means that Moshe and every future Bais Din is charged sanctifying Yom Tov as a special day. How does Bais Din do that? One way is by fixing a calendar which demarcates certain days as special holidays. Another way is by dedicating those days to special activities – to studying the Torah laws of those days and reading Torah parshiyos that are special.
Based on this idea, we better understand the role of learning hilchos Yom Tov. There are two separate functions to studying hilchos Yom Tov: firstly, fulfilling the mitzvah of talmud Torah; secondly, fulfilling the mitzvah of “vaYidaber Moshe es moadai Hashem el Bnei Yisrael”, of creating and enhancing kedushas Yom Tov. The obligation to study the halachos 30 days before Yom Tov is purely a function of the mitzvah of talmud Torah, to know the halachos. The obligation to study halacha on Yom Tov itself fulfills that added dimension of “vaYidaber Moshe es moadai Hashem el Bnei Yisrael”.