A special mass is held on Pascua de Pentecostés (Pentecost Sunday), and the church is decorated in red fifty days after Easter. The red decor is used primarily as a representation of the tongues of fire that appeared over 120 heads Jesus’ followers on Pentecost as described in the bible in Acts 2:1-31. The tongues of fire were believed to be manifestations of God’s holy spirit and bestowed the ability to speak in “tongues” (other languages) to those gathered. Those that had received the holy spirit in this form were charged with spreading the teachings of Christ throughout the world.
It is always observed on a Sunday, so as not to interfere with the work-week. May has a plethora of Mexican holidays already. This year (2015) the mass is Sunday, May 24.
Catholics are supposed to fast the evening before and there is a novena (9-day prayer) beginning El Jueves de la Ascension and ending on Pascua de Pentecostés (See La Novena).
As a high holy day, Pentecost Sunday is the big First Communion and Confirmation day. I suppose the idea is that you will be especially blessed by making your Catholicism known on this day. That little extra blessedness might just make the difference down the line. Better to safe than sorry right?
May is quite the month here in Mexico. Every time you turn around there is another celebration! For other Mexican May holidays see: El Día de los Trabajadores, Conmemoración del Escuadron de Pelea 201, El Dia de La Santa Cruz y El Dia del Albañil, La Batalla de Puebla, Natalicio de Miguel Hidalgo, El Dia de la Madre, El Jueves de la Ascensión, Pascua de Pentecostés, El Día del Maestro, and El Dia del Estudiante