Pentecost Monday & Pentecost Tuesday as Holy Days of Obligation

When writing about the rank of days in the Catholic Liturgical calendar, there are various ways to label them. In the modern Church, they will use the terms solemnity, feast, memorial, or optional memorial. In the 1962 Missal, we have First, Second, Third, or Fourth Class feastdays. But before the 1962 Missal up until the changes made by Pope Pius XII in 1955, there were from least to most important: Simples, Semidoubles, Lesser Doubles or also known as Doubles, Greater Doubles, Doubles of the second class, and lastly Doubles of the first class.   Using the traditional pre-1955 calendar, we notice something very interesting about Whit Monday and Whit Tuesday. Both of these days, like their counterparts in the Easter Octave, are doubles of the first class whereas the rest of the Pentecost Octave is of Double rank. The Wednesday, Friday, and Saturday of the Pentecost Octave are Ember Days and days of fasting and abstinence. 
Why the special treatment for Monday and Tuesday in the Octave of Pentecost? 

It is because they were universal holy days of obligation for a very long time. In 1642, Pentecost Monday and Pentecost Tuesday were listed as Holy Days per Universa Per Orbem. In 1771, Pope Clement XIV abolished both Easter Tuesday and Pentecost Tuesday as days of refraining from servile work. By 1778, they ceased being days of obligatory Mass attendance. Pentecost Monday was dropped from the universal list in 1911 by Pope St. Pius X's significant reduction in Holy Days on the Universal Calendar.

The Monday after Pentecost is still a holiday in Antigua and Barbuda, Anguilla, Austria, The Bahamas, Barbados, Belgium, The British Virgin Islands, Cyprus, Denmark, Dominica, France, Germany, Greece, Grenada, Hungary, Iceland, Ireland, Ivory Coast, Luxembourg, Monaco, Montserrat, The Netherlands, Norway, Romania, Saint Lucia, Saint Kitts and Nevis, Saint Vincent and the Grenadines, Solomon Islands, Switzerland, Togo, and Ukraine. Until 1973, it was also a holiday in Ireland, and until 1967 it was a bank holiday in the United Kingdom. And Sweden also continued to observe it as a public holiday until 2004.

The Catholic Encyclopedia affirms:

The office of Pentecost has only one Nocturn during the entire week. At Terce the "Veni Creator" is sung instead of the usual hymn, because at the third hour the Holy Ghost descended. The Mass has a Sequence, "Veni Sancte Spiritus" the authorship of which by some is ascribed to King Robert of France. The colour of the vestments is red, symbolic of the love of the Holy Ghost or of the tongues of fire. Formerly the law courts did not sit during the entire week, and servile work was forbidden. A Council of Constance (1094) limited this prohibition to the first three days of the week. The Sabbath rest of Tuesday was abolished in 1771, and in many missionary territories also that of Monday; the latter was abrogated for the entire Church by Pius X in 1911. Still, as at Easter, the liturgical rank of Monday and Tuesday of Pentecost week is a Double of the First Class.

While the Octave of Pentecost may not be days of precept anymore, we can certainly in our own prayer lives observe the Octave of Pentecost, hear Mass these days, pray the Divine Office more regularly, observe the Ember Days, and strive to sanctify them so as to do more than the minimum required by Church Law.

Veni Sancte Spiritus!