According to the Tzlach, when did the Ben Bs'seira/Hillel story happen?

The Tzlach says in Pesachim 66a that the safek of the bnei bseira was when or not to be me’aber the year in order that erev pesach doesn’t fall on Shabbos. This is mashmah that this happened around rosh chodesh nissan. but later in explaining tosafos d”h tochav he says (to answer the maharsha’s question) that they were makdish the knives on Shabbos Erev Pesach and the gemara says kol hayom kulo (only that day). So when did this happen?