The episode proper starts with a cold open. Walder Frey has gathered is kin together for a feast and is addressing them in a grandiose fashion. Those who remember the episode recap from 15 seconds earlier will realise this is Arya in disguise.
Fauxlder Frey declares that they all deserve a feast, the second this fortnight, because of how awesome they are. He even bought them special "Arbor Gold Wine" to mark the occasion. Except the women. No fancy plonk for them.
As the men drink, Fauxlder congratulates them on having the guts to kill the Starks at the Red Wedding. Even though they were guests in their house. And some were mothers. And pregnant.
It's around this time that the Freys begin to choke and keel over. As this is playing out, Fauxlder scolds them over not killing all of the Starks.