Last year, Easter was at the end of April, so El Jueves de la Ascensión (The Ascent of Christ) was May 29, forty days after Easter Sunday. This year, Easter was earlier. Therefore, this day is marked on May 14th. It always falls on a Thursday.
This religious event has been celebrated since the 5th century by the Catholic church. As Mexico remains predominantly Catholic, albeit its own mestizo version of Catholicism, El Jueves de la Ascension is a primary religious holiday.
It concludes the 40-day Lent period (See Carnival, Lent, Holy Week and Pilgrimages) and is based on the bible accounts found at Luke 24:50-52, Acts 1:9-11 and Mark 16:19. In these accounts, Jesus appeared to his 11 remaining disciples at the Mount of Olives near Jerusalem, spoke with them, and then was transfigured before their eyes, rising as a spirit and ascending to heaven.
The Feast of the Ascension is marked with an octave rather than a novena (See Novena) which would be eight days of prayers with possible fasting and is right up there with Easter and Pentecost in the list of high holy days.
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