The first day of the month of May, May first, is also known as May Day when referring to the traditional celebration that takes place on this day and which goes back to ancient pagan spring festivals celebrated by many northern hemisphere cultures around this time. The term May Day, which is always written with two capitalized words, goes back to the 13th century. In Spanish, both the date and the holiday can be expressed as either primero de mayo or uno de mayo, though the former is more common, and it is typically written with capitals, as Primero de Mayo, unlike regular dates, since it refers to a festivity and is thus treated as a proper name.
The division of the year into four periods with the actual dates set by astronomical dates in absolute terms, the equinoxes and solstices, was used primarily by the Celts in Europe and they only correspond approximately with actual weather conditions, which vary from place to place, depending mostly on latitude (distance from the tropics). The division of the year into four periods or seasons that we use today comes to us from the Romans (Lat. tempora anni ‘times of the year’), though many other ancient cultures, including the Chinese (which are also in the northern hemisphere) divide the year into four seasons as well. In many places, meteorological seasons are reckoned instead near the tropics, where the differences between the seasons may not be associated with temperature changes and with changes in agricultural practices as much as in more northern latitudes. It is common in these latitudes to divide the year into two seasons, typically called the rainy season and the dry season, which is what we find in many Spanish-speaking countries close to the Equator, where the former is known as invierno ‘winter’ and the latter as verano ‘summer’. In India, six seasons are distinguished and in Ancient Egypt, three.
As is well known, the varying temperatures during the seasons have to do with the varying tilt of the Earth towards the sun. The Sun’s rays hit the Earth at the most direct angle during the summer equinox and at the least direct angle during the winter equinox. The further away from the tropics, the more noticeable the temperature differences are. Actually, the most extreme hot or cold temperatures come after those dates, due to something known as the seasonal lag, which has to do with the large amount of heat or coldness retained in oceanic water. The lag may be of 2-3 weeks at the poles and as much as 12 weeks in lower latitudes. So, although spring starts officially around March 21, the change to consistent warmer weather doesn't start to be felt in the northern latitudes until the beginning of May, which is why the date of May 1st makes sense as the time to celebrate the beginning of the warm season or summer.
As Europe became Christianized in the Middle Ages, the pagan holidays that celebrated May Day lost their traditional pagan religious overtones and became merely popular celebrations, typically with religious overtones. As usual, there were Christian attempts to associate these traditional celebrations with Christian devotions, such as those associated with the Virgin Mary. They are known as Marian devotions (Sp. devociones marianas) and they are held during this whole month that eventually came to honor the Virgin Mary as the Queen of May. The traditions linking the Virgin Mary to the month of May go back many centuries, but they became more widespread in Catholic Europe only during the 19th century, culminating in official papal proclamations in the 20th century such as Pope Pius XII’s proclamation of the Queenship of Mary through his encyclical, Ad Caeli Reginam. In Protestant countries, on the other hand, cult of the Virgin Mary was greatly diminished since most Protestant Christian denominations came to see veneration and devotion of the Virgin Mary as Mariolatry, a form of idolatry.
The May Day traditions are strongest in the northern European countries of Germanic and, particularly, Celtic extraction, such as Scotland, Wales, Ireland, and England, but also in southern ones, such as Italy and Greece, and in many regions of Spain, especially the Celtic ones, such as Galicia in the north. Today, these traditions have receded to a great extent, however, and are mostly associated with rural areas in the countryside. In Catholic countries, such celebrations are much less important for example than those associated with Easter that take place usually in the month of March or early April (cf. §126.96.36.199). However, the Festividad de los Mayos ‘Festivity of the Mays’, also known as Los Mayos ‘The Mays’ or Fiestas de Mayo ‘May Holidays’, and the Fiesta de las Cruces ‘Holiday of the Crosses’, are still celebrated in many places in the Spanish-speaking world and they no doubt have their origins in ancient pagan May Day festivities.
Before leaving this festivity, we should mention a couple more issues related to May days and celebrations. One has nothing to do with the month of May, though it might appear to. We are referring to the English word mayday. This word refers to an international emergency procedure which is used as a distress signal in radio communications. The mayday procedure dates from 1923 and its source is the French phrase m’aider ‘(to) help me’, which is a shortened form of venez m’aider ‘come help me’. This was the voice equivalent of the earlier Morse code SOS message used in radio-telegraph communications for obtaining assistance.
Finally, we should mention the famous Mexican celebration of May 5, or Cinco de Mayo, which is well known by many in the United States as well, since it is a day in which Mexican-Americans traditionally celebrate their culture and Mexican heritage. Although some think that this day has to do with Mexican Independence, it is actually a commemoration of the Mexican Army’s unlikely victory over the French Empire at the Battle of Puebla, on May 5, 1862, a date that is not a national holiday in Mexico, though schools do close on that day. Mexico’s Independence Day is September 16, which is a national holiday. It commemorates the Cry of Dolores, a proclamation in the town of Dolores, that started Mexico’s war of independence from Spain in 1810. Cinco de Mayo started to be commemorated in California and other places in the western US with large Mexican populations soon after the battle as a celebration of Mexican cultural pride. In modern times, however, the holiday has been commercialized and trivialized to a great extent by the liquor industry and public schools, much like St. Patrick’s Day, which celebrates Irish heritage and culture.
 The original Spanish version has: ‘Árbol o palo alto, adornado de cintas, frutas y otras cosas, que se ponía en los pueblos en un lugar público, adonde durante el mes de mayo concurrían los mozos y mozas a divertirse con bailes y otros festejos’.