It says at the end of Parshas Sheleach:
דַּבֵּר אֶל בְּנֵי יִשְׂרָאֵל וְאָמַרְתָּ אֲלֵהֶם וְעָשׂוּ לָהֶם צִיצִת עַל כַּנְפֵי בִגְדֵיהֶם לְדֹרֹתָם וְנָתְנוּ עַל צִיצִת הַכָּנָף פְּתִיל תְּכֵלֶת
"Speak to the children of Israel and you shall say to them that they shall make for themselves fringes on the corners of their garments, throughout their generations, and they shall affix a thread of sky blue [wool] on the fringe of each corner."
Rashi says there on the words, פתיל תכלת:
על שם שכול בכורות. תרגומו של שכול תכלא. ומכתם היתה בלילה וכן צבע התכלת דומה לצבע רקיע המשחיר לעת ערב. ושמונה חוטים שבה, כנגד שמונה ימים ששהו ישראל משיצאו ממצרים עד שאמרו שירה על הים
"A thread of sky-blue [wool]: Heb. פְּתִיל תְּכֵלֶת, so called because of the bereavement [suffered by the Egyptians] over the loss of their firstborn. The Aramaic translation of שִׁכּוּל, bereavement, is תִּכְלָא [a word similar to תְּכֵלֶת]. Moreover, the plague struck them at night, and the color of תְּכֵלֶת is similar to the color of the sky, which blackens at dusk; its eight threads symbolize the eight days that Israel waited from when they left Egypt until they sang the song at the [Red] Sea. - [Mid. Aggadah]"
Rashi says that the 8 threads of the tzitzis correspond to the 8 days that Yisrael waited after leaving Mitzriam until singing shira. Where does this come from? Why specifically the connection between the 8 days and the 8 strings of the tzitzis? (I realize much could be said in the realm of drush, I'm more asking in the realm of p'shat as Rashi brings this.) What significance is there to this?
(I realize also there is a famous question here since in fact it was only 7 days until klal yisrael waited until they sang shiur. That question doesn't have to be mentioned as I'm trying to address a different point in this Rashi.)