Shiva and the festivals
Shabbos counts but doesn't stop it; a regel stops it
Since the Churban: Shavuos?
Rabbi EliezerLike Shabbos — it does not interrupt→ like Shabbos: shiva goes on
Once, six makeup days for its korbanos
Rosh Hashanah and Yom Kippur?
Rabban GamlielLike regalim — they interrupt→ like the regalim: they stop shiva
Chachamim: neither — Shavuos like a regel…
Rabbi EliezerLike Shabbos — it does not interrupt→ like Shabbos: shiva goes on
ChachamimLike a regel — it interrupts→ like a regel: it stops shiva
…Rosh Hashanah and Yom Kippur like Shabbos
Rabban GamlielLike regalim — they interrupt→ like the regalim: they stop shiva
ChachamimLike Shabbos — they do not interrupt→ like Shabbos
Halachah: all of them stop shiva
Moed Katan 3:6
Festivals and Mourning
הַמּוֹעֲדִים וְהַפְסָקַת הָאֲבֵלוּת
רַבִּי אֱלִיעֶזֶר אוֹמֵר, מִשֶּׁחָרַב בֵּית הַמִּקְדָּשׁ, עֲצֶרֶת כְּשַׁבָּת.
רַבָּן גַּמְלִיאֵל אוֹמֵר, רֹאשׁ הַשָּׁנָה וְיוֹם הַכִּפּוּרִים, כָּרְגָלִים.
וַחֲכָמִים אוֹמְרִים, לֹא כְדִבְרֵי זֶה וְלֹא כְדִבְרֵי זֶה,
אֶלָּא עֲצֶרֶת כָּרְגָלִים, רֹאשׁ הַשָּׁנָה וְיוֹם הַכִּפּוּרִים כְּשַׁבָּת.