What are the implications of "v'Shamarta"?
Menachos 36b: Wherever the Torah uses the terms "Hishamer," "Pen," or "Al," it implies a Lav -- even the "Hishamer" of an Aseh 1 (such as here). Consequently, whoever wears Tefilin after nightfall (refer to 13:10:1:2) transgresses a Lav.
What is ... l'Mo'adah, mi'Yamim Yamimah" coming to teach us?
Rashi, R. Bachye #1, Rashbam and Targum Onkelos 1 : With reference to the Korban Pesach, it teaches us to observe it in its given time - once a year. 2
R. Bachye #2 (citing R. Chananel): It is a warning to keep a Cheshbon from one year to the next 3 - a further proof that the months are fixed mainly according to the Cheshbon and not according to the witnesses' testimony that they saw the new moon. 4
R. Bachye #3 and Hadar Zekenim (citing Midrash Tanchuma, according to Shamai): It teaches us that one must check one's Tefilin every year. 5 6
R. Bachye #4 (citing Pesikta Zutr'sa), Targum Yonasan, and Menachos 36b (according to R. Yossi ha'Gelili): With reference to the Mitzvah of Tefilin, it teaches us 7 to wear them on working days, but not on Shabbos and Yom-Tov; 8 and to wear them by day but not by night. 9
See Na'ar Yonasan.
Like we find in Shoftim 11:40 & 21:19.
See R. Chavel's footnotes.
Refer to 12:2:3.1:1.
R. Bachye: Which we learn via a Gezeirah Shavah "mi'Yamim Yamimmah" "Yamim Tih'yeh Ge'ulaso" (Vayikra 25:39) from Bateo Arei Chomah.
Hadar Zekenim: According to Hillel, it is not necessary; because the Tefilin that his father left him are [presumed] Kasher.
R. Bachye: "le'Mo'adah" implies that on some days one wears Tefilin and on some days one doesn't.
See also Ba'al ha'Turim, and Torah Temimah note 50. Refer also to 13:9:1.1:1, and see Na'ar Yonasan, who cites the Machlokes Rebbi Yossi ha'Gelili and R. Akiva in Eruvin (96a) and elaborates. See also Moshav Zekenim.
Moshav Zekenim: The B'nei Eretz Yisrael maintain that this Pasuk comes to prohibit wearing Tefilin at night; therefore they recite[d] a B'rachah upon removing the Tefilin at the end of the day. The B'nei Bavel, on the other hand, hold that this Pasuk refers to the Pesach (see answer #1); and they do not recite such a B'rachah. R. Yeshayah asked, if so, what is the B'nei Bavel's source to forbid Tefilin at night? And why don't they recite the B'rachah "Lishmor Chukav" after eating Matzah? (Perhaps they hold that mid'Rabanan, one may not wear Tefilin at night! And why did R. Yeshayah ask about after eating Matzah? They hold that the Chok here is [Korban] Pesach! And "Veshamarta" refers to guarding against sinning (such as not to wear Tefilin at night)! Perhaps he means that which we are careful to eat Matzah, and not Chametz - PF).
