On Daf 19b the phrase Bo Bayom is used to reference the day 18 disputes between Beis Hillel and Beis Shamai were discussed and decided in favor of Beis Hillel. However in Brachos (I forgot exactly where) it's mentioned that expression is always used to refer to the day Raban Gamliel was deposed, more students were allowed in and lots of issues were decided.
How is the contradiction explained?
David Makowsky, Chicago, United States of America
Dear David,
Bo Bayom in Mishnayos means only the day R' Elazar Ben Azarya became Nasi of the Sanhedrin. See Mishnayos Yadayim 1:1 and Bartenura.
But the phrase Bo Bayom can also be used as "on the very same day" for other happenings: for the 18 Gezeros- Shabbos 15a,153b; for Hillel being appointed Nasi (Pesachim Tosefta 4); Adam HaRihon the day he was created (Avos d'R' Noson 1:8), etc.
Here also Bo Bayom means simply that the Gezeros of Yadayim and Keilim were on he same day.
All the best,
Reuven Weiner