Why does the Torah change here to "ve'NiskeiheM" from "ve'Niskah" on the other days, to "u'Nesachehah" on the sixth day, and on the seventh, from "ka'Mishpat" to "ke'MishpataM"?
Rashi (on Pasuk 18, citing the Sifri) and R. Bachye (on Pasuk 18, citing Ta?anis 2b): The three extra letters spell 'Mayim' - a hint to the Nisuch ha'Mayim ceremony, which began on the second day. 1 2
And was performed on six days, until the seventh day of Sukos. See Torah Temimah, note 20, who discusses the reason for Nisuch ha'Mayim.
The Si?man begins on the second day, because it was on the second day of the creation that Hashem divided the waters - See Bereishis 1:6. It extends to the sixth day, on which Adam, who needed water, was created, and to the seventh day, which concluded the creation. See R. Bachye who elaborates further.
QUESTIONS ON RASHI
Rashi writes that "ve'Niskeihem?" hints to Nisuch ha'Mayim is only on the sixth day. Why did the Torah not write it explicitly, like it wrote Nisuch ha'Yayin?
Moshav Zekenim: Nisuch ha'Yayin costs money, so the Torah needed to explicitly teach it. Nisuch ha'Mayim does not cost money, so a hint suffices.
