1. There is a dispute about whether preparations for a Bris that will be performed on Shabbos override Shabbos.
2. Rebbi Eliezer says that if one must carry the knife for the Bris through Reshus ha'Rabim, he should carry it uncovered.
3. The Gemara notes that there was a city that followed the opinion of Rebbi Eliezer (1 and 2 above).
4. Any Mitzvah that Bnei Yisrael accepted with joy is still done with joy today.
5. Any Mitzvah that was upheld carefully by Bnei Yisrael even during times of persecution remains carefully kept to this day.
A BIT MORE
1. Rebbi Eliezer says that they do. Rebbi Akiva says that only what cannot be done before Shabbos (such as the Milah itself, which must be done on the eighth day) overrides Shabbos.
2. This is for the sake of people who see him carrying the Bris knife in Reshus ha'Rabim on Shabbos. They will recognize that the Mitzvah of Bris Milah is so special that one is permitted even to carry in Reshus ha'Rabim to perform a Bris on Shabbos.
3. It also notes that no one in this city died before his destined time, but only when he was old. They merited longevity because they followed the rulings of their Rav, even when their Rav ruled leniently. (See Ritva in the name of the Ra'avad.)
4. The Gemara gives the example of the Mitzvah of Milah which was accepted with great joy (as is apparent from a verse in Tehilim). As a result, we continue to celebrate this Mitzvah with joy by making a festive meal in honor of the occasion.
5. The Gemara's examples of such Mitzvos are Bris Milah and the prohibition against Avodah Zarah.