Pesach, the seder, and the Exodus are full of “halves” and things that are “split”.
Avraham split the offerings in two for the Brit Bein Habetarim which ceremony prophecies the slavery and redemption of the Jews in Egypt; and it is referenced in the Haggadah.
The splitting of the Yam Suf.
The splitting of the matzah for Yachatz.
Makat bechorot happened exactly at chatzot- midnight or the point at which one can “split” the the night into two “halves”.
Pesach is celebrated on the 15th of the month of Nissan, the mid-point of the month. (One might justifiably think that we should observe Pesach on Rosh Chodesh; the beginning of the month very neatly can symbolize the beginning of Jewish history. If you are not persuaded by that question, remember the short paragraph from the Haggadah, “Yachol meRosh Chodesh…”! (Interesting that on the 15th, the moon is full.)
Many things happened on the fifteenth. Mechilta: Vayehi miketz- When the ketz-end came, Hashem did not prevent it from arriving like the blink of an eye. On the 15th of Nissan Hashem spoke to Avraham at BBH; the angels came to announce the birth of Yitzchak; Yitzchak was born; and the decree of BBH also occurred on the 15th of Nissan, ketz echad l’kulam, one “end” for them all.
The Hallel is split, two paragraphs during Magid and the rest after the meal. R”Y ben Yakar says as follows: One should not recite the brachah “ligmor et haHallel” because we say Shefoch chamascha as a hefsek between the parts of Hallel. All of the other elements of the seder that interrupt the two paragraphs of Hallel such as the brachah Asher Ga’alanu, the eating, the bentsching… can be viewed as part of the entirety of the evening and therefore not a hefsek. Keivan shecholkim oto ba’emtza, ein mevorchim ligmor. Since we split it in the “middle”…
According to many sources, the mitzvot of the seder including Hallel, teaching halachah, telling the story of the Exodus, eating the korban pesach in ancient times and the afikomen today,- all of these are required to be completed before chatzot.
Midrash Lekach Tov- Shmot 6:9- Mikotzer ruach ume’avodah kashah. The 430 years of servitude are calculated by adding the letters of M’K’TZ’R’. 40+100+90+200=430. These 430 years (shanah) consist of 210 years (shanim) for they are comprised of summer and winter. Shanah is the shinui, change of summer and shinui, change of winter. This midrash comes to explain the discrepancy between the number 430 and 210. The Torah states that the bondage of Egypt lasted 210 years but originally predicted 430 years. (Actually, in BBH the passuk says that Bnei Yisrael would be oppressed for 400 years. Mechilta explains it was 30 years until Yitzchak was born.) The numbers 430m and 210 are harmonized by dividing by 2. (Ok, 430/2=215, close enough!)
Actually this discrepancy is resolved in Pirkei D’Rabi Eliezer. Shmot 12:41- “… thirty years, and four hundred years”. Rabbi El’azar stated that one verse indicates that the Jews settled in Egypt for 210 years. And here the verse says 430. It was 400 years from the birth of Yitzchak, 210 years that they dwelled there, and 5 years until Yaakov came to Egypt and the Menashe and Efrayim were born. 215 days and nights make 430. (There’s your -5 error!) Dileg et haketz.
Yerushalmi Pesachim 10:5- Vayhi b’etzem hayom hazeh: Until where does one say Hallel (at the seder)? Beis Shamai says until “em habonim s’meichah” (first paragraph). Beis Hillel says until “chalamish l’mayno mayim” (second paragraph). Beis Shamai said to them, “Had Bnei Yisrael already left Egypt that the Exodus is mentioned?” (The second paragraph begins, B’Tzeis Bnei Yisrael Mimitzrayim.) Beis Hillel replied, “Some waited until the call of the rooster at which time even half of the redemption had not been reached. But hadn’t they only left at noon –b’chatzi hayom- as it says, b’etzem hayom, on that day? Rather since they began the mitzvah, we say to him finish. Beis Shamai holds that one eats the pesach only until midnight and then says B’tzeit Yisrael, for they hold the redemption and the exodus occurred at midnight. Beis Hillel holds that the key point is the “end” of the exodus which was in the morning.
I suggest that the theme of “half” is significant because Pesach is about redemption, but we can never feel that it is “complete”. The second “half” of the seder points to the final redemption. If we could sit back and relax, prematurely feeling that all was right with the world, the message of Passover would be muted. We need to think emphasize “half” because we “have not” accomplished the geulah shelemah.