Every year Thursday & Friday are off before Easter. "Semana Santa"
That Wednesday is the Argentine equivalent to the Wednesday before Thanksgiving in the USA. The bus station is absolutely slammed and everyone is travelling around the country.
Two years ago it took me two hours for our bus to get from Retiro to the entrance ramp for the highway near the base of Puerto Madero. That's about 2 miles. That's a long time. Just an FYI. (Though there might have been fog on the highway slowing everything down... I can't remember exactly.)