Who knows sixty-six?
Please cite/link your sources, if possible. At some point in the next few days, I will:
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Yerushalmi Sheviis (end of 5:1) says that when a tree develops, it takes 60 days for the first 6 leaves to grow, and then only 6 more days for the next 60 leaves to grow.
שמואל אמר שיתין יומין שיתא עליי שיתא יומין שיתין עליי
Thus, it takes 66 days for 66 leaves to emerge!
Following two weeks of tameh flow, a woman who gives birth to a girl has a 66 day period of tahor flow (Parashas Tazria)
When Yaakov went down to Mitzrayim, the descendants who came along with him numbered 66. (Bereishis 46:26)
66 are the degrees of longitude from the "western edge of civilization" to Jerusalem. (In the Rambam's times only the Eastern Hemisphere was known to be inhabited, so they spoke only of 180° of longitude, centered on a line corresponding to about modern 59° east of Greenwich. Jerusalem is 24° west of that line.)
(Commentaries to Rambam, Hil. Kiddush Hachodesh 11:17)
66 C.E. was the beginning of the Jewish revolt against the romans, which ultimately led to the destruction of the Temple: First Jewish–Roman War (Wikipedia).
(Well, at least according to a historical account. We know that the Temple was destroyed because of sinas hinam, but the Romans acted out the decree, and used the revolt as justification.)
Sefer Yeshayahu (the 2nd longest Sefer in Tenach, after Tehilim) has 66 perakim.