What is the significance of the seven days mentioned here?
Rashi: Seven days was the duration of each of the ten plagues, 1 with three weeks' warning in between. 2
Ramban: This refers to the period of time the Egyptians spent digging around the river, searching for water.
Rashbam and Targum Yonasan: The plague of blood lasted seven days. After seven days, Hashem cured the water of the river.
Moshav Zekenim: 'Nezifah' (censure) is not less than seven days (Mo'ed Katan 16a). Moshav Zekenim (to 19:1) -Hashem actually killed the Bechoros in one moment; however, it was seven days from when the Bechoros began killing their fathers.
Da'as Zekenim and Hadar Zekenim, and Riva (to 10:23, citing the Midrash Tanchuma): There were thirty days between one Makah and the next, like the Gematriya of the extra 'Chaf-Yud' in "Shalach Osos u'Mofesim b'Sochechi Mitzrayim" (Tehilim 135:9). Choshech lasted six days, and the darkness at Keri'as Yam Suf completed the count of seven days.
QUESTIONS ON RASHI
Rashi writes: "'And it was filled' (va'Yimalei) - [i.e.] the count of seven days." Why does Rashi add the word "count"?
Gur Aryeh: The singular tense of this verb is difficult, because "seven days" is plural. Rashi explains that it is referring to "a count" of seven, in singular.
Rashi writes: "Because [each] Makah would be in force for a quarter of a month, and for [the preceding] three-quarters, he would adjure and warn them." Why does Rashi tell us this here?
Gur Aryeh: The term "va'Yimalei," "the time was filled," implies that there was a set duration that had to elapse (e.g., in the verse, "her days were filled to give birth" (Bereishis 25:24) - meaning the nine months of pregnancy). The Makos too had a set duration - 1/4 of a month.
Rashi writes: "... Each Makah would be in force for a quarter of a month." The Torah tells us that in the plagues of Tzefarde'a, Arov, Barad, and Arbeh (8:8, 8:26, 9:33, 10:18 respectively), Moshe prayed that the Makah should end. If seven days had already elapsed, why did Moshe have to pray? If seven days had not yet passed, then the Makah was not in force for its full duration!
Gur Aryeh (to 8:5) #1: Although the frogs ceased attacking upon Moshe's prayer, the plague lasted for seven days in the sense that even after the frogs died, "the land reeked" (8:10).
Gur Aryeh (to 8:5) #2: By that time, Pharaoh already realized that each Makah would last a full week. Even when a Makah was cut short by Moshe's Tefilah, Pharaoh lived in fear that it would return at any moment. 1 (In that sense, these Makos also lasted the full week.)
Maharal #1 (Gevuros Hashem beg. Ch. 33, p. 124): The Makah itself lasted seven days, yet its after-effects would have lingered far beyond, had Moshe not prayed. For example, for seven days the Nile constantly spouted up more and more frogs. Even afterwards, those frogs already created would have continued hopping about -- had Moshe not prayed. Similarly, Moshe's prayer caused the Arov and Arbeh to disappear, and that the Barad that was already in mid-air would not melt and drip down.
Maharal #2 (ibid.): Each plague was to last for seven days - provided that Pharaoh did not agree to send the Bnei Yisrael free. On the occasions when Pharaoh said that he would free them, this completed the Makah. Moshe's prayer was then effective in removing its effects - even during the seven days. 2
Although Gur Aryeh (loc. cit.) mentions Choshech as an example, it is unclear why. (Gur Aryeh is about to give a different answer about Choshech; see 7:25:1.4:1). In Gevuros Hashem, Maharal addresses the plague of Arbeh. (EK).
But prior to Makas Arbeh as well (10:8), and during Makas Choshech (10:24), Pharaoh spoke about freeing the Bnei Yisrael - so why didn't this stop the Makah? However, at these times, Pharaoh would not agree to Moshe's terms, of freeing even the Bnei Yisrael's children and their livestock. (During Choshech, he failed even to ask Moshe to pray). That is why Arbeh began; and why Makas Choshech continued for its intended duration. (CS)
Rashi writes: "... Each Makah would be in force for a quarter of a month." But Rashi (to 10:22) tells us that Makas Choshech lasted for only six days; why was Choshech different?
Gur Aryeh (to 8:5): According to the Midrash, the final day of Choshech was reserved for Keri'as Yam Suf, when the Mitzrim were enveloped in cloud and darkness (14:20). 1
But perhaps Gur Aryeh's answer above (that Pharaoh remained in fear even when a plague ended early - see 7:25:1.3:2) applies to Makas Choshech as well. (EK)
