The feast marks the day on which tradition places the last apparition to St. Juan Diego, December 12, 1531. December 9, the day of the first apparition, is kept as the feast of St. Juan Diego. Pilgrim groups walk and cycle in from across Mexico for days beforehand, many travel through the night of the 11th, and some cross the atrium on their knees.
The basilica's sacristy lists the two principal Masses of the feast at the main altar at midnight and at noon on December 12, and Masses continue through the night and the day; the full program is published by the basilica each year. The scale is the thing to plan around. The figure of about 13 million was given by the head of the city government, Clara Brugada, for the ten days of the city's security operation in 2025, up from more than 12 million the year before; the same operation put about 5,000 police officers and teams of ambulances around the sanctuary.
Good to know before you go: do not count on a taxi or coach reaching the entrance. Pilgrims commonly report that the avenues leading to La Villa are given over to people on foot on these days, so plan to walk the last stretch or to come by Metro, to La Villa-Basílica on Line 6 or Deportivo 18 de Marzo on Line 3, and expect packed trains. Carry water and very little else, agree on a meeting point, and book lodging far ahead if you want to stay within walking distance. Choose the feast because the celebration itself matters to you; for quiet prayer before the image, come a week or two earlier or later. Priests who wish to concelebrate must send their request to the sacristy by December 6.