The day on which the Conclave begins is ordinarily to be the fifteenth day after the death of a Pope, the 16th day of the Interregnum. However, the College of Cardinals is given the faculty by Universi Dominici Gregis to defer its beginning "for serious reasons" up to the 20th day after death (21st day of the Vacancy). It must begin on or before that day.

On the morning of the first day on which the Conclave is to begin, the Cardinal Electors gather in St. Peter’s Basilica, or another place as may be determined by the College, to celebrate a Votive Mass for the Election of the Pope.

In the afternoon they gather in the Pauline Chapel of the Apostolic Palace. Invoking the assistance of the Holy Spirit with the Veni Creator Spiritu, they process to the Sistine Chapel. There they take a solemn oath to observe the prescriptions of the law governing the election, to observe the secrecy obliged, to not assist any secular power which may try to influence the election. The also swear that if elected they will faithfully carry out the Petrine Office, and protect the spiritual and temporal rights of the Holy See.

After the last Cardinal Elector has taken the oath, the Master of Papal Liturgical Ceremonies gives the order Extra omnes, commanding everyone not authorized to remain in the Conclave to leave the Chapel. Besides the Electors, only the Master of Papal Liturgical Ceremonies and the ecclesiastic chosen beforehand to give a meditation to the Cardinals on the seriousness of their duties, remains. When the meditation has been concluded, both of these men depart the Sistine Chapel.

After the Cardinals recite prayers provided in the proper Ordo for the Conclave, the Cardinal Dean inquiries if any Electors have questions concerning the norms and procedures. Once these have been clarified, by a majority decision of the Cardinals the election can proceed.

Only the Cardinal Electors may remain in the Sistine Chapel during the actual voting, which by law is from after the ballots have been distributed until after they have been tabulated and checked. Outside of the time of actual voting, the Secretary of the College, the Master of Papal Liturgical Ceremonies and the 2 Masters of Ceremonies are present to assist the Conclave.

On this first day of the Conclave, only one ballot is permitted. On the other days of the Conclave, two ballots are permitted in the morning session and two are permitted in the afternoon session.