Chabad.org — English translation + Rashi
Torah Temimah (Pesachim 77a): “במועדו” — even on Shabbos; “במועדו” — even in a state of tumah (when the majority of the community is tamei).
English: The Sages taught: “And Moshe spoke the appointed times of the Lord” — what does this teach? Because we had only learned regarding the tamid and the Pesach, of which it is said “at its appointed time”: “at its appointed time” — even on Shabbos; “at its appointed time” — even in tumah.
Gezerah / overriding force of “be-mo’ado” — the fixed time of Pesach overrides both Shabbos and communal tumah under defined conditions.
The appointed time is so binding that ordinary barriers of Shabbos and tumah yield when the community as a whole is affected.