At the end of Parshas Shelach we find the following fundamental verse on tzitzit:

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 say to them that they shall make for themselves tassels on the corners of their garments, throughout their generations, and they shall attach a thread of sky blue [wool] on the fringe of each corner.”

On the words פתיל תכלת Rashi writes, “A thread of sky-blue [wool]: Heb. פְּתִיל תְּכֵלֶת, so called because of the bereavement [the Egyptians suffered] over the loss of every firstborn. The Aramaic translation of שִׁכּוּל, bereavement, is תִּכְלָא [which resembles the word תְּכֵלֶת]. Moreover, the plague struck them at night, and the color תְּכֵלֶת is similar to the color of the sky as it darkens at dusk; its eight threads symbolize the eight days that Israel waited between the time they left Egypt until they sang the song at the [Red] Sea.”

Rashi says that the eight tzitzit strings correspond to the eight days the Jewish people waited after leaving Egypt until singing the song of praise.

For a thread on this topic, see Mi Yodeya.