Sede Vacante, Sede Vacante

However, after twenty days have elapsed, they must hold the conclave even if cardinals are missing. Historically, sede vacante periods have often been quite lengthy, lasting many months due to lengthy deadlocked conclaves. For many years through 1922 the period from the death of the Pope to the start of the conclave was shorter, but after William Henry Cardinal O'Connell had arrived just too late for two conclaves in a row, Pope Pius XI extended the time limit. With the very next conclave in 1939, cardinals began to travel by air.

Source: Wikipedia > Sede Vacante