What is the meaning of "Misas Nidvas Yadcha"?
Rashi: It means that one should bring Shalmei Simchah according to the B'rachah that Hashem has bestowed upon him 1 - the more one can afford, the more Shelamim one should bring, and invite people to eat them. 2
Rashbam: "Misas" means 'enough' - and the Torah is teaching us that need it is enough to give according to one's means and it is not necessary to go beyond that.
Chagigah, 8a: The word "Misas" intrisically implies Chulin - like we find in Esther,10:1 "Vayasem ha'Melech Achashverosh Mas" - to teach us that one should bring one's Korb'nos ha'Chag from Chulin and not from Ma'ser Sheini money. 3
Refer also to 16:17:1:1*.
The word "Nidvas" implies that one should give in accordance with one's generosity.
Why does the Torah insert the phrase "Ka'asher Yevarech'cha Hashem Elokeha"?
Chagigah, 8a: With reference to 16:10:1:2, it teaches us that if the owner wants to mix in some money of Ma'aser Sheini, he may do so. 1
See Torah Temimah, citing Yerushalmi Chagigah, 1:2 for a similar, more detailed D'rashah. Ses notes 45 & 46.
Seeing as Pesach and Sukos last seven and eight days respectively, why is Shavu'os only one day?
Sifri (in Pasuk 15): Because, as opposed to Pesach and Sukos, which fall on days that are not busy work days, it falls duting the harvest season where everybody is busy working in the fields - to teach us that the Torah has compasion on the money of Yisrael.
