What is the significance that tomorrow is Rosh Chodesh?
Rashi: Everyone who eats at the king's table comes on the Mo'ed. 1
Pesachim 77a: Rosh Chodesh is called Mo'ed - "Kara Alai Mo'ed" (Eichah 1:15).
What is the meaning of "Yipaked Moshavecha"?
Rashi: The place where you sit will be lacking (empty). Radak - this is like "v'Lo Nifkad Mimenu Ish" (Bamidbar 31:49).
How did he know that tomorrow will be Rosh Chodesh? Perhaps witnesses of the new moon will not come!
R. Bechayei, Abarvenel (Shemos 12:2, both citing R. Chananel): This shows that Rosh Chodesh was fixed based on calculation [of when the Molad will be]. It was so from the days of Moshe until Antigonus. 1 Also refer to 20:27:1:2*.
Rambam (Hilchos Kidush ha'Chodesh 8:4), Shibolei ha'Leket (Rosh Chodesh 168 citing Rashi), Tur (OC 427) : We always call the 30th day Rosh Chodesh, even when the month is full and we count the coming month from day 31, for part of day 30 pertains to the new month. I.e. they always celebrated the 30th of the month, and when witnesses did not come, they treated also the second day as Rosh Chodesh (and it was truly Rosh Chodesh - Tiferes Yisrael, Shevilei ha'Raki'a, Kevi'as ha'Chodesh 7).
His Talmidim Tzadok and Baisus said that seeing it is primary; Chachamim refuted them. (Why does the Torah permit witnesses of the new moon to be Mechalel Shabbos (Rosh Hashanah 1:4) if it is not the primary Mitzvah? And why did Chachamim begin to rely on witnesses only after rebels said that witnesses are primary? And if Rosh Chodesh was based on calculation, why did the Yevanim object to it more than Yom Tov, Shemitah and Yovel? This is unlike Pesachim 77a, which says that Hashem caused Tamuz (when the Meraglim went) to be full, so they would return on Tish'ah b'Av! Rabanan (Shabbos 87b) say that Iyar after Yetzi'as Mitzrayim was full, so Sivan 1 was on Monday! (PF)
