תענית ב' ב'
nd day, the Torah says םונסכיה other days says ונסכה extra letter מ
6th day, the Torah says היונסכ other days says ונסכה extra letter י
th day, the Torah says ם כמשפט other days says כמשפט extra letter ם
(סוכה נ' ב') explains that this Mitzvah of blessing the rain of the year during Sukkos (the beginning of the rain season) is connected to the six days of creation.
Rabbi Epstein explains that the world was created in Tishrei, and in Tishrei we begin to mention the power of rain. And in the beginning the ability to have life comes from the separation of the waters when land appeared. Thus, on Sukkos we commemorate with the pouring of the water, with Hallel, and with song, that G-d would bless us with rain in its proper time, just like G-d established in the beginning of the world.
According to this understanding, the allusion to the pouring of the water on Sukkos is specifically mentioned on the second, sixth and seventh days, to remind us that the nature of water was mentioned in the second, sixth and seventh verses of Creation (as cited above).
Further, we find support for the halacha that it is acceptable if a final Mem is written in the middle of a word in place of the standard Mem, since in the above word for Mayim the Torah uses twice a final Mem.
