The Mishna on our daf states: Anyone who has not seen the simhat bet ha-sho’eva – the joyousness of the water libation ceremony – has not seen true joy in their lifetime.
The Mishna describes how the Sages would sing songs and juggle torches, accompanied by an orchestra of levi’im, all to the light of large candelabra, which were large bowls of oil lit by the young kohanim who climbed ladders to do so.
Another impressive sight described by the baraita was the great synagogue in Alexandria, Egypt, which held double 600,000 people (the number 600,000 is significant because it was the number of people who left Egypt to come to Israel). The synagogue was so large that someone was appointed by the congregants to stand on a wooden platform in the middle and wave a flag so that everyone would know when it was time to respond “Amen” to the hazzan. Furthermore, every guild had its own section in the synagogue so that when a stranger would come, he could find his fellow tradesmen who would help support him and his family.
With this grand introduction, the Gemara concludes by quoting Abaye, who says that this entire community was destroyed by Alexander Mokdon because of their disregard for the passage forbidding Jews to return to Egypt (see Devarim 17:16).
In the Jerusalem Talmud, Rabbi Shimon bar Yohai is quoted as saying that the prohibition against returning to Egypt appears three times in the Torah, and when Jews returned for the third time, their fate was sealed. The Maharsha connects this with the story at the end of Sefer Yirmiyahu, where the prophet not only forbids the people from leaving Israel and going to Egypt, but also tells them that if they choose to return they will be killed by sword, starvation and disease (Yirmiyahu 42:17).
It should be noted that Alexander Mokdon cannot possibly be the general who wiped out the Jewish community in Alexandria, something already pointed out by the rishonim. The story apparently refers to the Roman Caesar Targenos (Trajan), who put down a Jewish rebellion against Rome about 60 years after the destruction of the Second Temple.