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, Rashbam and Targum Onkelos 1 : With reference to the Korban Pesach, it teaches us to observe it in its given time - once a year.
Targum Yonasan, and Menachos 36b (according to R. Yossi ha'Gelili): With reference to the Mitzvah of Tefilin, it teaches us to wear them on working days, but not on Shabbos and Yom-Tov; 2 and to wear them by day but not by night. 3
Hadar Zekenim (citing Midrash Tanchuma, according to Shamai): It teaches us that one must check one's Tefilin every year. 4
See Na'ar Yonasan.
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. Moshav Zekenim - Why don't the Bnei Eretz Yisrael (who apply this Pasuk to Tefilin, see the following note) learn from the word "l'Mo'adah," that Tefilin must be worn even on Shabbos [like we learn from the word "b'Mo'ado" in connection with the Korbenos Tzibur - Menachos 72b]? [Rather,] it teaches that Tefilin is only six days of the week, and not on Shabbos. (Perhaps he means that without the Pasuk, we would say that the Mitzvah applies always, therefore the Pasuk teaches us that the Mitzvah applies only at certain times - PF. Because donning Tefilin is not a Melachah, our initial assumption would be that the Mitzvah applies on Shabbos, were it not for this Pasuk. Offering Korbanos involves Melachah, so we must assume they are not brought on Shabbos, were it not for the Pasuk "b'Mo'ado." Also, perhaps there is a difference between "l'Moadah" with a Lamed, and "b'Mo'ado" with a Beis - CS.)
Moshav Zekenim: The Bnei Eretz Yisrael maintain that this Pasuk comes to prohibit wearing Tefilin at night; therefore they recite[d] a Berachah upon removing the Tefilin at the end of the day. The Bnei Bavel, on the other hand, hold that this Pasuk refers to the Pesach (see answer #1); and they do not recite such a Berachah. R. Yeshayah asked, if so, what is the Bnei Bavel's source to forbid Tefilin at night? And why don't they recite the Berachah "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 "v'Shamarta" refers to guarding against Aveirah (such as not to wear Tefilin at night)! Perhaps he means that which we are careful to eat Matzah, and not Chametz - PF).
Hadar Zekenim: According to Hillel, this is not necessary; because the Tefilin that his father left him are [presumed] Kasher.
