Vyāsa-pūjā is only one day, the Ekādaśī after Rāma-navamī, I think that is the 23rd of April. For some reason, somebody without my knowledge, said Vyāsa-pūjā is three days, 22nd to 24th, but I don’t know why they said that. But according to ISKCON rules, only one day is allowed for Vyāsa-pūjā day.
Since we fast on Ekādaśī from grains, cereals, pulses, some devotees want to have a feast. So day after they offer a feast, but that is not a part of the Vyāsa-pūjā celebration. A day before, devotees prepare themselves mentally for the Vyāsa-pūjā, but that is also not a part of the Vyāsa-pūjā celebration. Vyāsa-pūjā is only on one day, so I don’t know who is responsible for saying it is three days, but they should change that immediately.