From section 37:
from the moment when the Apostolic See is lawfully vacant, the Cardinal electors who are present must wait fifteen full days for those who are absent; the College of Cardinals is also granted the faculty to defer, for serious reasons, the beginning of the election for a few days more. But when a maximum of twenty days have elapsed from the beginning of the vacancy of the See, all the Cardinal electors present are obliged to proceed to the election.
