When did they gather?
Malbim: It was before they prepared provisions and sent messengers [to Binyamin - verse 12].
Why does it say "Kol Ish Yisrael... Chaverim"?
Chagigah 26a: This teaches that during the festival, when all of Yisrael gather, they are all considered Chaverim (trustworthy about Taharah). 1
Rashi: They all had the same counsel 2 (Radak - to avenge the ones who did this abomination).
Kli Yakar (Lani'ado) #1: They felt friendship also to Binyamin, and from them they will eradicate the evil.
Kli Yakar #2: The ones who prepared the food did not complain, and say 'we wanted to fight.'
Maharsha (26a), Torah ha'Temimah: If so, it should be whenever they are gathered, even not during the Regel! Piskei ha'Rid (26a) implies that it is whenever they gather. (PF) Meromei Sadeh (26a) - all Yisrael gather only during the Regel. Mishneh l'Melech (Hilchos Tum'as Ochlim 16:10) - The Rambam holds that we are lenient, lest people come to feud; Chachamim found a reason to permit, for everyone is Metaher himself before the festival. Rashi (Beitzah 11b): It is to avoid shaming Amei ha'Aretz.
